gitlab: clean-up build-soft-softmmu job

gitlab: clean-up build-soft-softmmu job

Having dropped alpha we also now drop xtensa as we don't have the
compiler in this image. It's not all doom and gloom though as a number
of other targets have gained softmmu TCG tests so we can add them. We
will take care of the other targets with their own containers in
future commits.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20231029145033.592566-5-alex.bennee@linaro.org>