clutter/master-clock: Remove fallback throttles

The presentation timing logic (via `master_clock_get_swap_wait_time`) now
works unconditionally. By "works" we mean that a result of zero from
`master_clock_get_swap_wait_time` actually means zero now. Previously
zero could mean either a successful result of zero milliseconds or that
the backend couldn't get an answer. And a non-zero result is the same as
before.

This works even if the screen is "idle" and even if the backend doesn't
provide presentation timestamps. So now our two fallback throttling
mechanisms of relying on `CLUTTER_FEATURE_SWAP_THROTTLE` and decimating
to `clutter_get_default_frame_rate` can be deleted.

Closes: GNOME/mutter#406 and
        https://bugzilla.gnome.org/show_bug.cgi?id=781835
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