Revert "gmain: Use alternate signal stack if the application provides one"
This reverts commit 280c8d41.
It breaks the unit tests on macOS (see #3314 (closed)) and no fix has been forthcoming.
The alternate stack changes can be resubmitted once they include a working unit test on macOS, as evidently its treatment of alternate stacks differs from that on Linux, and hence needs testing.
Helps: #3314 (closed)