x86/ftrace: Remove SYSTEM_BOOTING exceptions

Now that text_poke is available before ftrace, remove the
SYSTEM_BOOTING exceptions.

Specifically, this cures a W+X case during boot.

Signed-off-by: Peter Zijlstra (Intel) <peterz@infradead.org>
Link: https://lkml.kernel.org/r/20221025201057.945960823@infradead.org