CLEANUP: checks: fix double usage of cur / current_step in tcp-checks

CLEANUP: checks: fix double usage of cur / current_step in tcp-checks

This cleanup is a preliminary requirement to the upcoming fixes for
the bug that affect tcp-check's improper use of lists. It will have
to be backported to 1.5 though it will not easily apply.

There are two variables pointing to the current rule within the loop,
and either one or the other is used depending on the code blocks,
making it much harder to apply checks to fix the list walking bug.
So first get rid of "cur" and only focus on current_step.