Remove MergeRequest#target_branches and its usages

update_branches controller action has been removed in
244f511e

Now we can remove the view as well
The view was the only one which used @target_branches instance
variable, and this variable was the only one which used
MergeRequest#target_branches

The commit also remove the usage of MergeRequest#source_branches
Since we don't use that method anymore, we can delete it as well

app/controllers/projects/merge_requests/creations_controller.rb

@@ -16,7 +16,6 @@ class Projects::MergeRequests::CreationsController < Projects::MergeRequests::Ap
   end
 
   def create
-    @target_branches ||= []
     @merge_request = ::MergeRequests::CreateService.new(project, current_user, merge_request_params).execute
 
     if @merge_request.valid?
@@ -97,13 +96,6 @@ class Projects::MergeRequests::CreationsController < Projects::MergeRequests::Ap
 
   def define_new_vars
     @noteable = @merge_request
-
-    @target_branches = if @merge_request.target_project
-                         @merge_request.target_project.repository.branch_names
-                       else
-                         []
-                       end
-
     @target_project = @merge_request.target_project
     @source_project = @merge_request.source_project
 

app/controllers/projects/merge_requests_controller.rb

@@ -357,7 +357,6 @@ class Projects::MergeRequestsController < Projects::MergeRequests::ApplicationCo
   def define_edit_vars
     @source_project = @merge_request.source_project
     @target_project = @merge_request.target_project
-    @target_branches = @merge_request.target_project.repository.branch_names
     @noteable = @merge_request
 
     # FIXME: We have to assign a presenter to another instance variable

app/models/merge_request.rb

@@ -1129,26 +1129,6 @@ class MergeRequest < ApplicationRecord
     end
   end
 
-  # Return array of possible target branches
-  # depends on target project of MR
-  def target_branches
-    if target_project.nil?
-      []
-    else
-      target_project.repository.branch_names
-    end
-  end
-
-  # Return array of possible source branches
-  # depends on source project of MR
-  def source_branches
-    if source_project.nil?
-      []
-    else
-      source_project.repository.branch_names
-    end
-  end
-
   def has_ci?
     return false if has_no_commits?
 

app/views/projects/merge_requests/creations/update_branches.html.haml

-= render 'projects/merge_requests/dropdowns/branch',
-branches: @target_branches,
-selected: nil