Remove registertrying option in chan_sip

Remove registertrying option in chan_sip

This option is not only useless, but has been broken since inception since
the flag was never copied from the peer where it is set to the pvt where
it was checked. RFC 3261 specificially states that you should not send a
provisional response to a non-INVITE request, and if we did fix the code
so that it worked, it would cause the same kind of user enumeration
vulnerability that we've discussed with the nat= setting. This patch
removes registertrying option and any code that would have sent a 100
response to a register.

Review: https://reviewboard.asterisk.org/r/1562/
........

Merged revisions 343220 from http://svn.asterisk.org/svn/asterisk/branches/1.8

git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/10@343221 65c4cc65-6c06-0410-ace0-fbb531ad65f3