x86/spec-ctrl: Drop the bti= command line option

x86/spec-ctrl: Drop the bti= command line option

bti= was introduced with the original Spectre fixes (Jan 2018), but by the
time Speculative Store Bypass came along (May 2018), it was superceeded by the
more generic spec-ctrl=.

Since then, we've had LazyFPU (June 2018) and L1TF (August 2018), which means
noone will be using the option.  Remove it entirely - anyone who happens to
accidentially be using it might now spot Xen complaining about an option it
doesn't understand.

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