Remove skip-with-block feature.

This was initially intended as an replacement for `pending` with a block
(which has also been removed), but it's still really confusing.
Behaviour was the same as commenting out the code, which isn't useful.

@myronmarston sorry for a third PR, but I didn't want discussion of this to block the other bug-fix. I'm going to check 2.99 to see if any deprecation messages need to be changed.

cc https://github.com/yujinakayama/transpec/issues/38