rules/control-structᵒ: Fold two rules together

There were two rules that meant to remove unnecessary (do)s from
function definitions: one for functions with, and one for functions
without docstrings. These were folded together into a single rule.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>
rules/control-structᵒ: Make the (do) removal stricter

When trying to remove unnecessary (do)s from a function, make sure that
if there is something before the do, it really is a docstring.

Signed-off-by: Gergely Nagy <algernon@madhouse-project.org>