x86/nospec: Remove unused RSB_FILL_LOOPS

x86/nospec: Remove unused RSB_FILL_LOOPS

It was never really used, see

  117cc7a908c8 ("x86/retpoline: Fill return stack buffer on vmexit")

  [ bp: Massage. ]

Signed-off-by: Anthony Steinhauser <asteinhauser@google.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Cc: "H. Peter Anvin" <hpa@zytor.com>
Cc: Ingo Molnar <mingo@redhat.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: Thomas Gleixner <tglx@linutronix.de>
Cc: x86-ml <x86@kernel.org>
Link: https://lkml.kernel.org/r/20191226204512.24524-1-asteinhauser@google.com