tests: remove test-gdbstub.py

This isn't directly called by our CI and because it doesn't run via
our run-test.py script does things slightly differently. Lets remove
it as we have plenty of working in-tree tests now for various aspects
of gdbstub.

Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Signed-off-by: Alex Bennée <alex.bennee@linaro.org>
Message-Id: <20230829161528.2707696-7-alex.bennee@linaro.org>