My goodness, haven't handled an extension deletion. Add code to...

My goodness, haven't handled an extension deletion. Add code to ast_context_remove_extension2() to remove an extension from the trie. Done by marking it deleted. The scoreboard won't update for it any more. Also, a couple of calls to insert hashtab had a spurious ->exten, which was removed.

git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@89346 65c4cc65-6c06-0410-ace0-fbb531ad65f3