This removes a special case where `pymqi` was depended on by all target
platforms (including macOS on x86_64/AMD64) *except macOS on
AArch64/ARM64*.

There doesn't seem to be a compelling reason for keeping such an ad hoc
exclusion since `pymqi` 1.12.11 basically consists in an installer that
extracts a source tarball (https://pypi.org/project/pymqi/#files) and
compiles it for the _effective_ platform, in place.

In other words, what works for `macosx_10_12_x86_64` should also work
for `macosx_11_0_arm64`.

Moreover, `pymqi` is declared as an **optional** project dependency for
both "IBM ACE" and "IBM MQ", which already offers some leverage since
the dependency is _not installed unless explicitly qualified_:
- https://setuptools.pypa.io/en/latest/userguide/dependency_management.html#optional-dependencies
- https://pydevtools.com/handbook/explanation/what-are-optional-dependencies-and-dependency-groups/#optional-dependencies-end-user-features

Should unforeseen consequences happen despite above cautions, then
further actions may be considered (alternatively to a trivial rollback):
1. either drop the dependency at all for macOS where not needed (that is
   on all macOS architectures),
2. and/or bump IBM MQ deliverables to a version that better accomodates
   the AArch64/ARM64 architecture on macOS: #20645.