Pull CEL linkedid manipulation into cel.c

This finishes moving all CEL linkedid tracking entirely within cel.c
since that is now possible with channel snapshots.

This also removes another CEL linkedid manipulation function from cel.h
that has already been internalized and is neither called nor available
to link against.

Review: https://reviewboard.asterisk.org/r/2632/


git-svn-id: http://svn.asterisk.org/svn/asterisk/trunk@392241 f38db490-d61c-443f-a65b-d21fe96a405b