Reuse iterator when erasing callbacks.
Reuse iterator when erasing callbacks.

Summary: Doing an extra lookup here is wasteful.

Reviewed By: yangchi, lnicco

Differential Revision: D20848227

fbshipit-source-id: 8a1fee28597fae3cf1ac284a1bd781936ddff931

facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst,facebookincubator/mvfst