Revert M1 heap_size workaround

Summary: Undo changes made in D703 as they're no longer necessary. It looks like it was an OS issue resolved in macOS 11.3 beta 3 (https://github.com/facebook/flow/issues/8538#issuecomment-795826130).

Test Plan: ran flow in each directory and it worked

Reviewers: ashoat, palys-swm

Reviewed By: ashoat

Subscribers: KatPo, zrebcu411, Adrian, subnub

Differential Revision: https://phabricator.ashoat.com/D896