includes/Stg.h: remove unused 'wcStore' inline

Summary:
Commit reverts never used addition in cbd29e0a23bb8e15033edae123d6c8fbe9740c97

I think it might make sense to take advantage of TSO/RMO/PSO models
tome day. But it's highly architecture/model-dependent thus it better
be implemented in per-arch Native CodeGen.

Signed-off-by: Sergei Trofimovich <slyfox@gentoo.org>

Test Plan: build-tested on UNREG-amd64

Reviewers: simonmar, austin

Reviewed By: austin

Subscribers: simonmar, ezyang, carter

Differential Revision: https://phabricator.haskell.org/D186