Explicitly tag :core and :mattermost scenarios

It's adding additional potentially confusing behaviour to derive test
tagging from the file path, as it's not immediately apparent where that
lives when considering the tests in isolation.  Instead, just explicitly
tag.

* Remove the tagging of examples based on :file_path
* Explicitly tags scenarios as :core, or :mattermost

qa/qa/specs/config.rb

@@ -33,14 +33,6 @@ module QA
             mocks.verify_partial_doubles = true
           end
 
-          config.define_derived_metadata(file_path: %r{/specs/features/(?!mattermost/)}) do |metadata|
-            metadata[:core] = true
-          end
-
-          config.define_derived_metadata(file_path: %r{/specs/features/mattermost/}) do |metadata|
-            metadata[:mattermost] = true
-          end
-
           config.order = :random
           Kernel.srand config.seed
           config.formatter = :documentation

qa/qa/specs/features/login/standard_spec.rb

 module QA
   feature 'standard root login' do
-    scenario 'user logs in using credentials' do
+    scenario 'user logs in using credentials', :core do
       Page::Main::Entry.act { sign_in_using_credentials }
 
       # TODO, since `Signed in successfully` message was removed

qa/qa/specs/features/mattermost/group_create_spec.rb

 module QA
   feature 'create a new group' do
-    scenario 'creating a group with a mattermost team' do
+    scenario 'creating a group with a mattermost team', :mattermost do
       Page::Main::Entry.act { sign_in_using_credentials }
       Page::Main::Menu.act { go_to_groups }
 

qa/qa/specs/features/project/create_spec.rb

 module QA
   feature 'create a new project' do
-    scenario 'user creates a new project' do
+    scenario 'user creates a new project', :core do
       Page::Main::Entry.act { sign_in_using_credentials }
 
       Scenario::Gitlab::Project::Create.perform do |project|

qa/qa/specs/features/repository/clone_spec.rb

 module QA
   feature 'clone code from the repository' do
-    context 'with regular account over http' do
+    context 'with regular account over http', :core do
       given(:location) do
         Page::Project::Show.act do
           choose_repository_clone_http
@@ -30,7 +30,7 @@ module QA
         end
       end
 
-      scenario 'user performs a deep clone' do
+      scenario 'user performs a deep clone', :core do
         Git::Repository.perform do |repository|
           repository.location = location
           repository.use_default_credentials
@@ -41,7 +41,7 @@ module QA
         end
       end
 
-      scenario 'user performs a shallow clone' do
+      scenario 'user performs a shallow clone', :core do
         Git::Repository.perform do |repository|
           repository.location = location
           repository.use_default_credentials

qa/qa/specs/features/repository/push_spec.rb

 module QA
   feature 'push code to repository' do
     context 'with regular account over http' do
-      scenario 'user pushes code to the repository' do
+      scenario 'user pushes code to the repository', :core do
         Page::Main::Entry.act { sign_in_using_credentials }
 
         Scenario::Gitlab::Project::Create.perform do |scenario|