x86: Remove FAST_FEATURE_TESTS

Since we want to rely on static branches to avoid speculation, remove
any possible fallback code for static_cpu_has.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Signed-off-by: Thomas Gleixner <tglx@linutronix.de>
Cc: torvalds@linux-foundation.org
Link: https://lkml.kernel.org/r/20180319154717.705383007@infradead.org