x86/mm: Remove unused current_untag_mask()

e0bddc19 ("x86/mm: Reduce untagged_addr() overhead for systems without LAM")

removed its only usage site so drop it.

Move the tlbstate_untag_mask up in the header and drop the ugly
ifdeffery as the unused declaration should be properly discarded.

Signed-off-by: Borislav Petkov (AMD) <bp@alien8.de>
Acked-by: Kirill A. Shutemov <kirill.shutemov@linux.intel.com>
Link: https://lore.kernel.org/r/20230614174148.5439-1-bp@alien8.de