chan_sip: No BYE message sent after INVITE with Replaces

Setting channel state DOWN is an unnecessary step that was
only being done in handle_invite_replaces().  This changes
that by removing the call and reducing locking.

(closes issue ASTERISK-23010)
Reported by: Ryan Tilton
Review: https://reviewboard.asterisk.org/r/3116/
........

Merged revisions 405486 from http://svn.asterisk.org/svn/asterisk/branches/1.8


git-svn-id: https://origsvn.digium.com/svn/asterisk/branches/11@405487 65c4cc65-6c06-0410-ace0-fbb531ad65f3