Use the common working copy lock pattern when removing externals. There should
be no noticable different behavior.

* subversion/libsvn_client/externals.c
  (remove_external2,
   remove_external): New functions, implementing the common lock obtain and
     release.
  (handle_external_item_removal): Use remove_external as cleaner way to ensure
     a lock. Drive the removal on the information in the store to avoid deleting
     unrelated nodes.
  (svn_client__handle_externals): Remove obsolete comment.


git-svn-id: https://svn.apache.org/repos/asf/subversion/trunk@1434705 13f79535-47bb-0310-9956-ffa450edef68