x86/fpu: Remove export of mxcsr_feature_mask

x86/fpu: Remove export of mxcsr_feature_mask

The variable was previously referenced in KVM code but the last usage was
removed by:

    ea4d6938d4c0 ("x86/fpu: Replace KVMs home brewed FPU copy from user")

Remove its export symbol.

Reviewed-by: Nikolay Borisov <nik.borisov@suse.com>
Signed-off-by: Chang S. Bae <chang.seok.bae@intel.com>
Signed-off-by: Ingo Molnar <mingo@kernel.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: H. Peter Anvin <hpa@zytor.com>
Cc: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Oleg Nesterov <oleg@redhat.com>
Link: https://lore.kernel.org/r/20250416021720.12305-10-chang.seok.bae@intel.com