The function isn't used since commit:

  5ad274d4 ("x86/irq: Remove unused old IOAPIC irqdomain interfaces")

Removing it fixes the following warning when building with clang:

  arch/x86/kernel/apic/io_apic.c:1219:19: error: unused function
      'IO_APIC_irq_trigger' [-Werror,-Wunused-function]
Signed-off-by: Matthias Kaehlcke <mka@chromium.org>
Cc: Andrew Morton <akpm@linux-foundation.org>
Cc: Douglas Anderson <dianders@chromium.org>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: http://lkml.kernel.org/r/20170522232035.187985-1-mka@chromium.orgSigned-off-by: Ingo Molnar <mingo@kernel.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>