um: Remove unused do_get_thread_area function

It's not used since it was introduced by commit aa6758d4 ("[PATCH]
uml: implement {get,set}_thread_area for i386"). Now, it's causing a
-Wmissing-prototypes warning:

arch/x86/um/tls_32.c:39:5: warning: no previous prototype for ‘do_get_thread_area’ [-Wmissing-prototypes]
   39 | int do_get_thread_area(struct user_desc *info)
      |     ^~~~~~~~~~~~~~~~~~

The original author also had doubts about whether it should be used.
Considering that 18 years have passed, let's just remove it.

Signed-off-by: Tiwei Bie <tiwei.btw@antgroup.com>
Signed-off-by: Richard Weinberger <richard@nod.at>