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
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Merged revisions 408005 from http://svn.asterisk.org/svn/asterisk/branches/12
git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@408006
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