Sample modules.conf: comment out example load statement.

The sample modules.conf explicitly loaded res_musiconhold.so.  This is
redundent as autoload=yes is already set.  It causes warnings if
res_musiconhold.so was not installed and results in an unexpected load
if the admin disables autoload without remembering to remove the
res_musiconhold load statement.

Also remove reference to unknown module pbx_gtkconsole.

Change-Id: Ib01888994d9f1364b14d3c9fb6ff96774a6e580a