hhvm: remove jemalloc profiling completely

Contrary to jemalloc's documentation, we are still seeing heap maps
being dumped in production and that causes both latency and occasional
puppet failures (due to our housekeeping job that should supposedly
disable those failing). So, comment out the MALLOC_CONF line in the
upstart script.

Change-Id: Ie99c8a43aab965aeaf5c6ef421c937275e78b6a8