This reverts commit 24fa20e5.

Although these symmetric variants sometimes look "better", they
are really annoying and should IMHO never be used:

1.) For lemmas there is now a choice between >= and <=. Since there is
no longer a canonical choice, it is very easy to introduce a lot of
inconsistencies in statements of lemmas.

2.) For automation the situation becomes annoying, you have to built in
stuff for both >= and <=. That is very error-prone.

3.) For N and Z the notions x <= y and y >= x are not even convertible!
That means that done/by does not solve x <= y if you have y >= x and if
avoids you applying certain lemmas.