futex: Remove unused variable 'vpid' in futex_proxy_trylock_atomic()

The recent bug fix left the variable 'vpid' and an assignment to it around,
but the variable is otherwise unused.

clang dose not complain even with W=1, but gcc exposed this.

Fixes: 4f07ec0d ("futex: Prevent inconsistent state and exit race")
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>