Merge branch 'remove-unused-templates-from-gitlab-ci' into 'master'

Clean up some unused templates imported from GitLab CI

## What does this MR do?

Clean up some unused templates imported from GitLab CI, for new developers to easily understand the file/directory structure of Gitlab CE.

## Are there points in the code the reviewer needs to double check?

Be sure if removed files are not used now while the committer checked them once.

## Why was this MR needed?

Because the file/directory structure of the repository should be clear and simple and if so new developers can easily understand the file/directory structure of Gitlab CE.

## What are the relevant issue numbers?

Closes #19158 

## Does this MR meet the acceptance criteria?

- CHANGELOG won't be updated due to repository maintenance
- No documentation created/updated
- No API support added
- Tests
  - [skipped] Added for this feature/bug
  - [x] All builds are passing
- [x] Conform by the [style guides](https://gitlab.com/gitlab-org/gitlab-ce/blob/master/CONTRIBUTING.md#style-guides)
- [x] Branch has no merge conflicts with `master` (if you do - rebase it please)
- [x] [Squashed related commits together](https://git-scm.com/book/en/Git-Tools-Rewriting-History#Squashing-Commits)

See merge request !4922

app/views/ci/errors/show.haml

-%h3.error Error
-= @error

app/views/ci/shared/_guide.html.haml

-.bs-callout.help-callout
-  %h4 How to setup CI for this project
-
-  %ol
-    %li
-      Add at least one runner to the project.
-      Go to #{link_to 'Runners page', runners_path(@project), target: :blank} for instructions.
-    %li
-      Put the .gitlab-ci.yml in the root of your repository. Examples can be found in
-      #{link_to "Configuring project (.gitlab-ci.yml)", "http://doc.gitlab.com/ci/yaml/README.html", target: :blank}.
-      You can also test your .gitlab-ci.yml in the #{link_to "Lint", ci_lint_path}
-    %li
-      Return to this page and refresh it, it should show a new build.

app/views/ci/shared/_no_runners.html.haml

-.alert.alert-danger
-  %p
-    Now you need Runners to process your builds.
-  %span
-    Checkout the #{link_to 'GitLab Runner section', 'https://about.gitlab.com/gitlab-ci/#gitlab-runner', target: '_blank'} to install it
-
-

app/views/layouts/ci/_info.html.haml

-- if current_user && current_user.is_admin? && Ci::Runner.count.zero?
-  = render 'ci/shared/no_runners'

app/views/layouts/ci/_page.html.haml

-.page-with-sidebar{ class: page_sidebar_class }
-  = render "layouts/broadcast"
-  .sidebar-wrapper.nicescroll{ class: nav_sidebar_class }
-
-    - if defined?(sidebar) && sidebar
-      = render "layouts/ci/#{sidebar}"
-    - elsif current_user
-      = render 'layouts/nav/dashboard'
-    .collapse-nav
-      = render partial: 'layouts/collapse_button'
-    - if current_user
-      = link_to current_user, class: 'sidebar-user', title: "Profile" do
-        = image_tag avatar_icon(current_user, 60), alt: 'Profile', class: 'avatar avatar s36'
-        .username
-          = current_user.username
-  .content-wrapper
-    = render "layouts/flash"
-    = render 'layouts/ci/info'
-    %div{ class: container_class }
-      .content
-        .clearfix
-          = yield

app/views/layouts/ci/notify.html.haml

-%html{lang: "en"}
-  %head
-    %meta{content: "text/html; charset=utf-8", "http-equiv" => "Content-Type"}
-      %title
-        GitLab CI
-
-  %body
-    = yield :header
-
-    %table{align: "left", border: "0", cellpadding: "0", cellspacing: "0", style: "padding: 10px 0;", width: "100%"}
-      %tr
-        %td{align: "left", style: "margin: 0; padding: 10px;"}
-          = yield
-          %br
-      %tr
-        %td{align: "left", style: "margin: 0; padding: 10px;"}
-          %p{style: "font-size:small;color:#777"}
-            - if @project
-              You're receiving this notification because you are the one who triggered a build on the #{@project.name} project.