Refactor force_remove_source_branch handling

In build_service.rb, let's move most of the force_remove_source_branch
handling code into a new force_remove_source_branch() method.

app/services/merge_requests/build_service.rb

@@ -16,13 +16,8 @@ module MergeRequests
       merge_request.source_project = find_source_project
       merge_request.target_project = find_target_project
 
-      # Source project sets the default source branch removal setting
-      merge_request.merge_params['force_remove_source_branch'] =
-        if params.key?(:force_remove_source_branch)
-          params.delete(:force_remove_source_branch)
-        else
-          merge_request.source_project.remove_source_branch_after_merge?
-        end
+      # Force remove the source branch?
+      merge_request.merge_params['force_remove_source_branch'] = force_remove_source_branch
 
       self.params = assign_allowed_merge_params(merge_request, params)
 
@@ -74,6 +69,14 @@ module MergeRequests
              :errors,
              to: :merge_request
 
+    def force_remove_source_branch
+      if params.key?(:force_remove_source_branch)
+        params.delete(:force_remove_source_branch)
+      else
+        merge_request.source_project.remove_source_branch_after_merge?
+      end
+    end
+
     def find_source_project
       source_project = project_from_params(:source_project)
       return source_project if source_project.present? && can?(current_user, :create_merge_request_from, source_project)