x86/speculation: Remove x86_spec_ctrl_mask

commit acac5e98 upstream.

This mask has been made redundant by kvm_spec_ctrl_test_value().  And it
doesn't even work when MSR interception is disabled, as the guest can
just write to SPEC_CTRL directly.

Signed-off-by: Josh Poimboeuf <jpoimboe@kernel.org>
Signed-off-by: Borislav Petkov <bp@suse.de>
Reviewed-by: Paolo Bonzini <pbonzini@redhat.com>
Signed-off-by: Borislav Petkov <bp@suse.de>
Signed-off-by: Thadeu Lima de Souza Cascardo <cascardo@canonical.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>