Fix memory leak in pbx_dundi

pbx_dundi added an io context without removing
it. This caused a memory leak when the module was
unloaded.

(closes ASTERISK-21718)
Reported by Corey Farrell
Patches:
	pbx_dundi-ast_io_remove.patch uploaded by Corey Farrell (License #5909)
........

Merged revisions 388376 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@388378 65c4cc65-6c06-0410-ace0-fbb531ad65f3