Remove unintended recursion in vec_delete.

Remove unintended recursion in vec_delete.

This prevents a segfault: Fixes #2225.

Add a little more documentation around the responsibilities of callers
to make this a little less likely to happen again.

I thought about calling this vec_vec_delete but went with cycles_delete;
the only place this is used is for cycles.

This effectively reverts c4ef19f483d577a8395a1a1f9ca5a321af3472b5,
and fixes that commit's memory leak in a different way.