x86: Remove gdbstub

x86: Remove gdbstub

In 13y of working on Xen, I've never seen seen it used.  The implementation
was introduced (commit b69f92f3012e, Jul 28 2004) with known issues such as:

  /* Resuming after we've stopped used to work, but more through luck
     than any actual intention.  It doesn't at the moment. */

which appear to have gone unfixed for the 20 years since.

Nowadays there are more robust ways of inspecting crashed state, such as a
kexec crash kernel, or running Xen in a VM.

This will allow us to clean up some hooks around the codebase which are
proving awkward for other tasks.

Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Acked-by: Jan Beulich <jbeulich@suse.com>