use destroy_all

lib/gitlab/import_export/members_mapper.rb

@@ -41,9 +41,7 @@ module Gitlab
       end
       def ensure_default_member!
-        @project.project_members.each do |member|
-          member.destroy
-        end
+        @project.project_members.destroy_all
         ProjectMember.create!(user: @user, access_level: ProjectMember::MASTER, source_id: @project.id, importing: true)
       end