Garbage-collect ACPI-safe timer and friends

In 2001 when the ACPI timer was introduced, it included code to check
for a bug present in some Pentium II and Pentium III chipsets; if the
bug was found to be present, ACPI-safe (which was slower but had a
workaround for the bug) would be used rather than ACPI-fast (which
read the same timer but without the workaround).

In a8b89dff6ac0 (September 2021) I disabled this check by default,
with a loader tunable available to re-enable it; I announced at the
time that it would go away in FreeBSD 15 if I didn't receive any
reports of problems. I have received no such problems, so this code
is now going away.

The debug.acpi.timer_test loader variable triggered a lengthy (in fact,
infinitely long) test of the ACPI timer and appears to have been
introduced as part of the process of writing the ACPI timer (and the
associated ACPI-safe workaround) in 2001; since we are dropping support
for systems with this ACPI bug, there is no need to keep that test code
either.