x86/entry: Remove now unused do_IRQ() declaration

do_IRQ() has been replaced by common_interrupt() in

  fa5e5c40

 ("x86/entry: Use idtentry for interrupts")

Remove its now unused declaration.

Signed-off-by: Hao Lee <haolee.swjtu@gmail.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20210103030834.GA15432@haolee.github.io