Remove unused `gitlab:generate_docs` Rake task

This was the only thing using the `sdoc` gem, which was blocking another
gem from updating.

Gemfile

@@ -260,9 +260,6 @@ group :development do
   gem 'better_errors', '~> 1.0.1'
   gem 'binding_of_caller', '~> 0.7.2'
 
-  # Docs generator
-  gem 'sdoc', '~> 0.3.20'
-
   # thin instead webrick
   gem 'thin', '~> 1.7.0'
 end

Gemfile.lock

@@ -663,9 +663,6 @@ GEM
     scss_lint (0.47.1)
       rake (>= 0.9, < 11)
       sass (~> 3.4.15)
-    sdoc (0.3.20)
-      json (>= 1.1.3)
-      rdoc (~> 3.10)
     seed-fu (2.3.6)
       activerecord (>= 3.1)
       activesupport (>= 3.1)
@@ -956,7 +953,6 @@ DEPENDENCIES
   sanitize (~> 2.0)
   sass-rails (~> 5.0.6)
   scss_lint (~> 0.47.0)
-  sdoc (~> 0.3.20)
   seed-fu (~> 2.3.5)
   select2-rails (~> 3.5.9)
   sentry-raven (~> 2.0.0)

doc/development/rake_tasks.md

@@ -42,14 +42,6 @@ To run several tests inside one directory:
 If you want to use [Spring](https://github.com/rails/spring) set
 `ENABLE_SPRING=1` in your environment.
 
-## Generate searchable docs for source code
-
-You can find results under the `doc/code` directory.
-
-```
-bundle exec rake gitlab:generate_docs
-```
-
 ## Generate API documentation for project services (e.g. Slack)
 
 ```

lib/tasks/gitlab/generate_docs.rake

-namespace :gitlab do
-  desc "GitLab | Generate sdocs for project"
-  task generate_docs: :environment do
-    system(*%W(bundle exec sdoc -o doc/code app lib))
-  end
-end
-