base: Remove support for the VTOC8 partitioning scheme

The removal of the sparc64 support in February 2020 obsoleted the
VTOC8 partitioning scheme as no other FreeBSD platform makes use
of it. Moreover, the code is bitrotting as nothing defines e. g.
LOADER_VTOC8_SUPPORT any more and, thus, should go now, too. With
this change, the following commits are reverted as far as VTOC8
is concerned and parts haven't already previously been deleted
along with prior sparc64 removals:
094fcb157d4c98211899cf09d06e2cf19149b7e0
a7d366e9589c95feda6f3bc78c59c6355d51f126
ba8d50d08b9df4e8213f9a6997ff6792ecebcd9b

The alignment example d9711c28efc4ec89ba5ea11f8fd63e9d0a7fc81b
added to the VTOC8 section of gpart.8 is folded into the MBR one.

This should finally conclude the deorbit of sparc64-specific bits.

We had joy, we had fun
we ran Unix on a Sun.
But that source and the song
of FreeBSD have all gone.

Credits to Michael Bueker for the original "Unix on a Sun" and Rod
McKuen for the "Seasons in the Sun" lyrics.

(cherry picked from commit 4ef1c6f75d25ba0c264ab274a1b449467a6ee817)