Drop unused mach time headers

Summary:

Now that we're no longer special-casing clock usage for MacOS, we're
not referencing anything defined in these headers.

This is a backport of Core PR18229

Depends on D8774

Test Plan:
ninja all check-all

@bot build-osx

Reviewers: #bitcoin_abc, majcosta

Reviewed By: #bitcoin_abc, majcosta

Subscribers: Fabien

Differential Revision: https://reviews.bitcoinabc.org/D8776