Revert "Beta: use MemcachedPhpBagOStuff if running under HHVM"

This reverts commit 6088ef6138351e6713eb5cb6d34abfe53b1489de.

Change-Id: I3cc3b6d3c4224bc3df9cc1f5d48a989d459d1008
Merge "Revert "Beta: use MemcachedPhpBagOStuff if running under HHVM""