Revert incorrect STM wakeup optimisation

Authored by osa1 on Sep 11 2018, 1:43 PM.

Description

Revert incorrect STM wakeup optimisation

Summary: (see the comments)

Reviewers: simonmar, bgamari, erikd

Reviewed By: simonmar

Subscribers: rwbarton, carter

Differential Revision: https://phabricator.haskell.org/D5144

(cherry picked from commit 36740b4c346c619e31d24d6672caa6f4f7fea123)