Remove the old sanity check of sigcontext offsets for NetBSD/i386

Remove the old sanity check of sigcontext offsets for NetBSD/i386

NetBSD switched to ucontext, back in 2003 and the sigcontext code
is no longer available for users, except for legacy compat layers.

This code was not available anyway as the pre-processor check
was probably never operational and buildable on NetBSD. The code
inside it does not compile.

Meanwhile, move the offset variable into the ifdef goards and avoid
the error about unused variable.

../../gdb/i386-bsd-nat.c: In function 'void _initialize_i386bsd_nat()':
../../gdb/i386-bsd-nat.c:347:7: error: unused variable 'offset' [-Werror=unused-variable]
  347 |   int offset;
      |       ^~~~~~
cc1plus: all warnings being treated as errors

gdb/ChangeLog:

	* i386-bsd-nat.c (_initialize_i386bsd_nat): Update.
	* i386-nbsd-tdep.c (i386nbsd_sc_reg_offset): Now static.