x86/uaccess: Remove unused __addr_ok() macro

This was caught while staring at the whole {set,get}_fs() machinery.

It's last user, the 32-bit version of strnlen_user() went away with

  5723aa99

 ("x86: use the new generic strnlen_user() function")

so drop it.

No functional changes.

Signed-off-by: Borislav Petkov <bp@suse.de>
Acked-by: Linus Torvalds <torvalds@linux-foundation.org>
Cc: Andy Lutomirski <luto@kernel.org>
Cc: Jann Horn <jannh@google.com>
Cc: Peter Zijlstra <peterz@infradead.org>
Cc: the arch/x86 maintainers <x86@kernel.org>
Cc: "Tobin C. Harding" <tobin@kernel.org>
Link: https://lkml.kernel.org/r/20190225191109.7671-1-bp@alien8.de