menuselect: Remove mxml from menuselect (Patch 2)

This is the second patch that adds menuselect to Asterisk trunk. The previous
commit (r418832) added menuselect along with mxml; this patch removes mxml
completely from Menuselect.

A subsequent patch will switch menuselect over to using libxml2, and make
libxml2 a required dependency for Asterisk.

ASTERISK-20703


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@418833 65c4cc65-6c06-0410-ace0-fbb531ad65f3