Summary:
This is a manual commit of this PR:
Retire InMemoryEnv in favor of MockEnv #2082
With MockEnv doing the same yet being more mature, InMemoryEnv is redundant.

Reviewed By: IslamAbdelRahman

Differential Revision: D5162323

fbshipit-source-id: 59fd0082a891dc99cc531e4da9d68bf891eae3f5