Revert "macOS: Prevent Xcode 9.3 build warnings"

Summary:
This reverts commit 4f0e8a5a102abe5ed647d9f09d69666f9589cdcd.

This should fix the broken Windows binaries in the current release candidate.

Depends on D3851

Test Plan: Fabien to manual test gitian binaries on this patch.

Reviewers: #bitcoin_abc, deadalnix, Fabien

Reviewed By: #bitcoin_abc, Fabien

Differential Revision: https://reviews.bitcoinabc.org/D3852