Remove all PJSIP MWI-specific use from our MWI code.

PJSIP has built-in MWI code that could be useful to some
degree, but our utilization of the API actually made our
code a bit more cluttered since we had to have special
cases peppered throughout.

With this change, we move to using the pjsip_evsub API
instead, which streamlines the code by removing special
cases.

Review: https://reviewboard.asterisk.org/r/3205
........

Merged revisions 408005 from http://svn.asterisk.org/svn/asterisk/branches/12


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@408006 65c4cc65-6c06-0410-ace0-fbb531ad65f3