x86/irq: Remove vector_used_by_percpu_irq()

Last user (lguest) is gone. Remove it.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Borislav Petkov <bp@alien8.de>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Steven Rostedt <rostedt@goodmis.org>
Link: http://lkml.kernel.org/r/20170828064956.201432430@linutronix.de


Signed-off-by: Ingo Molnar <mingo@kernel.org>