[X86] Remove x86-experimental-unordered-atomic-isel option and associated code

This option enables an experimental lowering for unordered atomics I worked
on a few years back.  It never reached production quality, and hasn't been
worked on in years.  So let's rip it out.

This wasn't a crazy idea, but I hit some stumbling block which prevented me
from pushing it across the finish line.  From the look of 027aa27d, that
change description is probably a good summary.  I don't remember the
details any longer.