Dont show 'Remove source brnach' checkbox for fork

Remove source button checkbox always try to remove branch in target
project. For fork source & target projects are differents - so we hide
checkbox from UI. After MR accepted you can easily remove branch with
'Remove branch' button that do remove branch call to valid project
Signed-off-by: NDmitriy Zaporozhets <dmitriy.zaporozhets@gmail.com>

app/views/projects/merge_requests/show/_mr_accept.html.haml

@@ -38,7 +38,7 @@
         .accept-group
           .pull-left
             = f.submit "Accept Merge Request", class: "btn btn-create accept_merge_request"
-          - if can_remove_branch?(@merge_request.source_project, @merge_request.source_branch)
+          - if can_remove_branch?(@merge_request.source_project, @merge_request.source_branch) && !@merge_request.for_fork?
             .remove_branch_holder.pull-left
               = label_tag :should_remove_source_branch, class: "checkbox" do
                 = check_box_tag :should_remove_source_branch

lib/gitlab/satellite/merge_action.rb

@@ -31,8 +31,9 @@ module Gitlab
             # push merge back to bare repo
             # will raise CommandFailed when push fails
             merge_repo.git.push(default_options, :origin, merge_request.target_branch)
+
             # remove source branch
-            if merge_request.should_remove_source_branch && !project.root_ref?(merge_request.source_branch)
+            if merge_request.should_remove_source_branch && !project.root_ref?(merge_request.source_branch) && !merge_request.for_fork?
               # will raise CommandFailed when push fails
               merge_repo.git.push(default_options, :origin, ":#{merge_request.source_branch}")
             end