Checking global variables here actually overwrote the previous substitution by

channel variables, and in any case, was redundant;
pbx_substitute_variables_helper ALREADY does substitution for global
variables.
(closes issue #13327)
 Reported by: pj


git-svn-id: https://origsvn.digium.com/svn/asterisk/trunk@162922 65c4cc65-6c06-0410-ace0-fbb531ad65f3