x86/fpu: Cleanup the on_boot_cpu clutter

Defensive programming is useful, but this on_boot_cpu debug is really
silly.

Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Signed-off-by: Borislav Petkov <bp@suse.de>
Link: https://lkml.kernel.org/r/20211015011538.665080855@linutronix.de