linux-user/signal.c: Remove unnecessary wrapper copy_siginfo_to_user

linux-user/signal.c: Remove unnecessary wrapper copy_siginfo_to_user

The function copy_siginfo_to_user() just calls tswap_siginfo(), so
call the latter function directly and delete the wrapper function.
The wrapper is actually misleading since it implies that the
semantics are like the kernel function with the same name which
copies the data to a guest user-space address. In fact tswap_siginfo()
just does data-structure conversion between two structures whose
addresses are host addresses (the copy to userspace is handled
in QEMU by the lock_user/unlock_user calls).

This also fixes clang complaints about the wrapper being unused
in some configs.

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