x86/entry: Get rid of two-phase syscall entry work

I added two-phase syscall entry work back when the entry slow path
was very slow.  Nowadays, the entry slow path is fast and two-phase
entry work serves no purpose.  Remove it.

Signed-off-by: Andy Lutomirski <luto@kernel.org>
Signed-off-by: Kees Cook <keescook@chromium.org>