x86/docs: Remove reference to syscall trampoline in PTI

Commit

  bf904d27 ("x86/pti/64: Remove the SYSCALL64 entry trampoline")

removed the syscall trampoline and instead opted to enable using the
default SYSCALL64 entry point by mapping the percpu TSS. Unfortunately,
the PTI documentation wasn't updated when the respective changes were
made, so bring the doc up to speed.

Signed-off-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Link: https://lore.kernel.org/r/20231102130204.41043-1-nik.borisov@suse.com