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



git-svn-id: http://svn.asterisk.org/svn/asterisk/branches/12@408005 f38db490-d61c-443f-a65b-d21fe96a405b