Remove SpinLockFree() and S_LOCK_FREE().

Remove SpinLockFree() and S_LOCK_FREE().

S_LOCK_FREE() is used by the test program in s_lock.c, but nobody
has voiced concerns about losing some coverage there.
SpinLockFree() appears to have been unused since it was introduced
by commit 499abb0c0f.  There was agreement to remove these in 2020,
but it never happened.  Since we still have agreement for removal
in 2026, let's do that now.

Reviewed-by: Fabrízio de Royes Mello <fabriziomello@gmail.com>
Reviewed-by: Andres Freund <andres@anarazel.de>
Discussion: https://postgr.es/m/aZX2oUcKf7IzHnnK%40nathan
Discussion: https://postgr.es/m/20200608225338.m5zho424w6lpwb2d%40alap3.anarazel.de