Removed User#project_relations

GitLab EE adds an extra relation that selects a "project_id" column
instead of an "id" column, making it very hard for this method to be
re-used in EE. Since using User#authorized_groups in
ProjectsFinder#all_groups apparently has no performance impact we can
just use it and keep everything compatible with EE.

app/finders/projects_finder.rb

@@ -53,7 +53,7 @@ class ProjectsFinder
   def all_projects(current_user)
     if current_user
       [
-        *current_user.project_relations,
+        current_user.authorized_projects,
         public_and_internal_projects
       ]
     else

app/models/user.rb

@@ -442,11 +442,6 @@ class User < ActiveRecord::Base
     Project.where("projects.id IN (#{projects_union.to_sql})")
   end
 
-  # Returns all the project relations
-  def project_relations
-    [personal_projects, groups_projects, projects]
-  end
-
   def owned_projects
     @owned_projects ||=
       Project.where('namespace_id IN (?) OR namespace_id = ?',
@@ -835,7 +830,9 @@ class User < ActiveRecord::Base
   private
 
   def projects_union
-    Gitlab::SQL::Union.new(project_relations.map { |r| r.select(:id) })
+    Gitlab::SQL::Union.new([personal_projects.select(:id),
+                            groups_projects.select(:id),
+                            projects.select(:id)])
   end
 
   def ci_projects_union