block: delete super ancient PC-XT driver for 1980's hardware

This driver was for the 8 bit ISA cards that were installed in
the PC-XT machines of 1980 vintage.  They supported the dual
ribbon cable MFM drives of 10-20MB capacity, and ran at a 3:1
interleave, giving performance on the order of 128kB/s.

By the introduction of the PC-AT (286) these controllers were
already scrapped in favour of 16 bit controllers with some onboard
RAM that could support a 1:1 interleave.

The git history doesn't show any evidence of runtime fixes that
would reflect active usage; instead just the usual tree-wide API
type changes/cleanups.  Going back to in-source changelogs, the
last "runtime" fix that is evident is something I did over a
dozen years ago[1] -- and even back then, the hardware was long
since unavailable, so that ancient fix was also not runtime tested.

The time is long overdue for this to get flushed, so lets get
rid of it before anyone wastes more time doing builds and sparse
checks etc. on long since dead code.

[1] http://lkml.indiana.edu/hypermail/linux/kernel/0102.2/0027.html

Signed-off-by: Paul Gortmaker <paul.gortmaker@windriver.com>