tcg/s390: Remove unused tcg_out_addi()

Remove the unused function tcg_out_addi() from the s390 TCG backend;
this brings it into line with other backends.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Acked-by: Richard Henderson <rth@twiddle.net>
Signed-off-by: Blue Swirl <blauwirbel@gmail.com>