revert default value of flag folly_memory_idler_purge_arenas
revert default value of flag folly_memory_idler_purge_arenas

Reviewed By: yfeldblum

Differential Revision: D27575037

fbshipit-source-id: fa5817ee3fe1eca99e83f26a13e43ec4a4a6e7cf

facebook/folly,facebook/folly,facebook/folly,facebook/folly,facebook/folly