remove unused translation

Author: wxiaoguang

options/locale/locale_en-US.ini

@@ -1681,7 +1681,6 @@ issues.pin_comment = "pinned this %s"
 issues.unpin_comment = "unpinned this %s"
 issues.lock = Lock conversation
 issues.unlock = Unlock conversation
-issues.lock.unknown_reason = Cannot lock an issue with an unknown reason.
 issues.lock_duplicate = An issue cannot be locked twice.
 issues.unlock_error = Cannot unlock an issue that is not locked.
 issues.lock_with_reason = "locked as <strong>%s</strong> and limited conversation to collaborators %s"