(Cleanup) fb_thrift_unserialize

Summary: Discovered while typing `i fb_unserialize` in `hphpd`. This function
doesn't exist. TAO has more lovely references to it, and to files that
don't exist. Fun.

Reviewed By: @markw65

Differential Revision: D1747154

Signature: t1:1747154:1418922414:ba13c725d7341bb493acc67c1398a0c3401a4808