x86/bugs: Remove uses of cpu_mitigations_off()

cpu_mitigations_off() is no longer needed because all bugs use attack
vector controls to select a mitigation, and cpu_mitigations_off() is
equivalent to no attack vectors being selected.

Remove the few remaining unnecessary uses of this function in this file.

Signed-off-by: David Kaplan <david.kaplan@amd.com>
Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Reviewed-by: Pawan Gupta <pawan.kumar.gupta@linux.intel.com>