gh-37210: unify alias substitute for subs
We unify the definitions as follows:
* `substitute` is a method in `Element` which calls `self.subs` and
passes all arguments along, and not defined anywhere else.
* `subs` is the method that should be overwritten in the various element
classes.
Dependencies:
#37143
URL: https://github.com/sagemath/sage/pull/37210
Reported by: Martin Rubey
Reviewer(s): Frédéric Chapoton
