arm64: ssbd: Drop #ifdefs for PR_SPEC_STORE_BYPASS

Now that we're all merged nicely into mainline, there's no need to check
to see if PR_SPEC_STORE_BYPASS is defined.

Signed-off-by: Will Deacon <will.deacon@arm.com>
Signed-off-by: Catalin Marinas <catalin.marinas@arm.com>