Remove the rasops24_erasecols and rasops24_eraserows functions from

Remove the rasops24_erasecols and rasops24_eraserows functions from
rasops(9).

We only build rasops24 functions on amd64 (for EFI support in QEMU) and
on armv7 and arm64, and we can safely use the generic functions on those
platforms, like we do for all other color depths. No other platform uses
rasops24, Miod explains why here [1].

No performance regression observed when testing with QEMU in EFI mode.

[1] https://marc.info/?l=openbsd-ppc&m=118664346819989&w=2

OK kettenis@