rtmutex: Remove unused argument from rt_mutex_proxy_unlock()

From: Thomas Gleixner <tglx@linutronix.de>

[ Upstream commit 2156ac19 ]
Nothing uses the argument. Remove it as preparation to use
pi_state_update_owner().

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Acked-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Cc: stable@vger.kernel.org
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>