Summary: forward_iterator_bench is not stable enough for build. Remove it for now.

Test Plan: Build it with both of CLANG and non-CLANG and make sure it builds.

Reviewers: rven, kradhakrishnan, anthony, yhchiang, igor, IslamAbdelRahman

Reviewed By: igor, IslamAbdelRahman

Subscribers: leveldb, dhruba

Differential Revision: https://reviews.facebook.net/D50991