x86-64, trampoline: Remove unused variable

Removed unused variable left over from development.

Reported-by: Brian Gerst <brgerst@gmail.com>
LKML-Reference: <AANLkTik6UJ680mWJcu_W+jerLcqPjwjvaXyxB1jAMaG0@mail.gmail.com>
Cc: Stephen Rothwell <sfr@canb.auug.org.au>
Cc: Rafael J. Wysocki <rjw@sisk.pl>
Cc: Matthieu Castet <castet.matthieu@free.fr>