gh-38199: Delete _check_rank_two method for Drinfeld modules
This method does nothing if the Drinfeld module has rank two, and raises
an exception otherwise. It was introduced in the first version of the
implementation, when some methods were only available for the rank two
case (e.g. `frobenius_trace`, `frobenius_norm`, etc). A `grep` reveals
that the method is never called, which makes it useless. Therefore, I
propose to remove it.
URL: https://github.com/sagemath/sage/pull/38199
Reported by: Antoine Leudière
Reviewer(s): David Ayotte
