x86/smpboot: Remove the last ICPU() macro

Now all is using the shiny new macros.

No code changed:

  # arch/x86/kernel/smpboot.o:

   text    data     bss     dec     hex filename
  16432    2649      40   19121    4ab1 smpboot.o.before
  16432    2649      40   19121    4ab1 smpboot.o.after

md5:
   a58104003b72c1de533095bc5a4c30a9  smpboot.o.before.asm
   a58104003b72c1de533095bc5a4c30a9  smpboot.o.after.asm

Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: Thomas Gleixner <tglx@linutronix.de>
Link: https://lkml.kernel.org/r/20200324185836.GI22931@zn.tnic