xen/wait: Drop vestigial remnants of TRAP_regs_partial

xen/wait: Drop vestigial remnants of TRAP_regs_partial

The preservation of entry_vector was introduced with ecf9846a6a20 ("x86:
save/restore only partial register state where possible") where
TRAP_regs_partial was introduced, but missed from f9eb74789af7 ("x86/entry:
Remove support for partial cpu_user_regs frames") where TRAP_regs_partial was
removed.

Fixes: f9eb74789af7 ("x86/entry: Remove support for partial cpu_user_regs frames")
Signed-off-by: Andrew Cooper <andrew.cooper3@citrix.com>
Reviewed-by: Jan Beulich <jbeulich@suse.com>