sh: Rip out broken kgdb thread support.

The kgdb thread support is woefully out of date (it predates
the pidhash), and needs a complete rewrite before it's useful
again. Just rip it out entirely.

Updating the unified kgdb stub is a more worthwhile endeavour
for anyone that happens to be interested in this, at present
it's just limping along.

Signed-off-by: Paul Mundt <lethal@linux-sh.org>