um: Cleanup compiler warning in arch/x86/um/tls_32.c

arch.tls_array is statically allocated so checking for NULL doesn't
make sense. This causes the compiler warning below.

Remove the checks to silence these warnings.

../arch/x86/um/tls_32.c: In function 'get_free_idx':
../arch/x86/um/tls_32.c:68:13: warning: the comparison will always evaluate as 'true' for the address of 'tls_array' will never be NULL [-Waddress]
   68 |         if (!t->arch.tls_array)
      |             ^
In file included from ../arch/x86/um/asm/processor.h:10,
                 from ../include/linux/rcupdate.h:30,
                 from ../include/linux/rculist.h:11,
                 from ../include/linux/pid.h:5,
                 from ../include/linux/sched.h:14,
                 from ../arch/x86/um/tls_32.c:7:
../arch/x86/um/asm/processor_32.h:22:31: note: 'tls_array' declared here
   22 |         struct uml_tls_struct tls_array[GDT_ENTRY_TLS_ENTRIES];
      |                               ^~~~~~~~~
../arch/x86/um/tls_32.c: In function 'get_tls_entry':
../arch/x86/um/tls_32.c:243:13: warning: the comparison will always evaluate as 'true' for the address of 'tls_array' will never be NULL [-Waddress]
  243 |         if (!t->arch.tls_array)
      |             ^
../arch/x86/um/asm/processor_32.h:22:31: note: 'tls_array' declared here
   22 |         struct uml_tls_struct tls_array[GDT_ENTRY_TLS_ENTRIES];
      |                               ^~~~~~~~~

Signed-off-by: Lukas Straub <lukasstraub2@web.de>
Acked-by: Randy Dunlap <rdunlap@infradead.org> # build-tested
Signed-off-by: Richard Weinberger <richard@nod.at>