CEL refactoring cleanup

This change removes AST_CEL_BRIDGE_UPDATE since it should no longer be
used because masquerade situations are now accounted for in other ways.

This also refactors usage of AST_CEL_FORWARD to be produced by a Dial
message which has been extended with a "forward" field.

(closes issue ASTERISK-21566)
Review: https://reviewboard.asterisk.org/r/2635/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@392829 f38db490-d61c-443f-a65b-d21fe96a405b