Remove gitlab:user:check_repos task

While reviewing this branch it was noted that this task was broken.
Given the limited value this task brings, and given the fact that much
of the old behaviour has been moved to be automatically fixed, removing
was a good option.

Suggestion in:
https://gitlab.com/gitlab-org/gitlab-ce/merge_requests/20806#note_91261001

changelogs/unreleased/zj-remove-git-rake-tasks.yml

 ---
-title: Remove git maintainance housekeeping tasks
+title: Remove gitlab:user:check_repos, gitlab:check_repo, gitlab:git:prune, gitlab:git:gc, and gitlab:git:repack
 merge_request: 20806
 author:
 type: removed

doc/administration/raketasks/check.md

-# Check Rake Tasks
+# Integrity Check Rake Task
 
 ## Repository Integrity
 
@@ -29,7 +29,7 @@ exactly which repositories are causing the trouble.
 ### Check all GitLab repositories
 
 This task loops through all repositories on the GitLab server and runs the
-3 integrity checks described previously.
+integrity check described previously.
 
 **Omnibus Installation**
 
@@ -43,33 +43,6 @@ sudo gitlab-rake gitlab:git:fsck
 sudo -u git -H bundle exec rake gitlab:git:fsck RAILS_ENV=production
 ```
 
-### Check repositories for a specific user
-
-This task checks all repositories that a specific user has access to. This is important
-because sometimes you know which user is experiencing trouble but you don't know
-which project might be the cause.
-
-If the rake task is executed without brackets at the end, you will be prompted
-to enter a username.
-
-**Omnibus Installation**
-
-```bash
-sudo gitlab-rake gitlab:user:check_repos
-sudo gitlab-rake gitlab:user:check_repos[<username>]
-```
-
-**Source Installation**
-
-```bash
-sudo -u git -H bundle exec rake gitlab:user:check_repos RAILS_ENV=production
-sudo -u git -H bundle exec rake gitlab:user:check_repos[<username>] RAILS_ENV=production
-```
-
-Example output:
-
-![gitlab:user:check_repos output](../img/raketasks/check_repos_output.png)
-
 ## Uploaded Files Integrity
 
 Various types of files can be uploaded to a GitLab installation by users.

lib/tasks/gitlab/check.rake

@@ -414,23 +414,6 @@ namespace :gitlab do
     end
   end
 
-  namespace :user do
-    desc "GitLab | Check the integrity of a specific user's repositories"
-    task :check_repos, [:username] => :gitlab_environment do |t, args|
-      username = args[:username] || prompt("Check repository integrity for username? ".color(:blue))
-      user = User.find_by(username: username)
-      if user
-        repo_dirs = user.authorized_projects.map do |p|
-          p.repository.path_to_repo
-        end
-
-        repo_dirs.each { |repo_dir| check_repo_integrity(repo_dir) }
-      else
-        puts "\nUser '#{username}' not found".color(:red)
-      end
-    end
-  end
-
   # Helper methods
   ##########################