tests: remove skip() as unused

skip() is a left-over from an old test harness design, the comment even
refers to automake. Calling skip() cannot do anything good anymore,
because it wouldn't print the skips in the TAP report, so it would
probably be considered a failure.

Delete this unused and nowadays incorrect function, so it doesn't
confuse people.

Signed-off-by: Pekka Paalanen <pekka.paalanen@collabora.com>