Drop profile_before_collect_callback, which never runs
Drop profile_before_collect_callback, which never runs

Summary:
With the changes from the previous diff, we will no longer ever attempt to
collect during merge, and therefore not attempt to collect within the callback
argument to `with_memory_profiling_lwt`.

I don't imagine this callback has fired since cancelable rechecks were enabled,
since merging will not create garbage until the transaction is committed.

Reviewed By: nmote

Differential Revision: D25280251

fbshipit-source-id: 70d606ce5400128e044426b8a6fec4d5151a05b0

facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,facebook/flow,mroch/flow,facebook/flow,facebook/flow,mroch/flow,mroch/flow,mroch/flow,mroch/flow,facebook/flow