linux-user/signal.c: Remove current_exec_domain_sig()

linux-user/signal.c: Remove current_exec_domain_sig()

Remove the function current_exec_domain_sig(), which always returns
its argument. This was intended as a stub for supporting the kernel's
exec_domain handling, but:
 * we don't have any of the other code for execution domains
 * in the kernel this handling is architecture-specific, not generic
 * we only call this function in the x86, ppc and sh4 signal code paths,
   and the PPC one is wrong anyway because the PPC kernel doesn't
   have this signal-remapping code

So it's best to simply delete the function; any future attempt to
implement exec domains will be better served by adding the correct
code from scratch based on the kernel sources at that time.

This change also fixes some clang warnings about the function being
defined but not used for some target architectures.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Signed-off-by: Riku Voipio <riku.voipio@linaro.org>