hhvm: disable internal stat collection

Since we have a few tricky issues to chase down, I'd rather disable stat
collection entirely for the time being. There is no shortage of high-value
stats that we're not doing anything serious with (MediaWiki errors is the prime
example), so we can live without these for now.

Some of the configuration keys don't exist yet, and the rest are liable to be
changed in the next month or two, so let's clear these variables before they
become voodoo.

This reverts commits 32b70f6d and bcde4d8f.

Change-Id: I80f86a1313109c5f2838659c4cdc191de4c5d8fd