NO-JIRA: Removed Makefile.PL from DISTCLEANFILES in Autotools.

The file used to be generated, but is not any longer. So the file never
needs to be deleted when cleaning up.

git-svn-id: https://svn.apache.org/repos/asf/qpid/trunk@1430464 13f79535-47bb-0310-9956-ffa450edef68