gh-37115: Remove check for deprecated attribute _no_generic_basering_coercion
In `sage.categories.unital_algebras.UnitalAlgebras`, remove the check
for the attribute `_no_generic_basering_coercion`, which was deprecated
in #19225. This means that this attribute is now completely ignored by
the coercion framework.
URL: https://github.com/sagemath/sage/pull/37115
Reported by: Peter Bruin
Reviewer(s): Marc Mezzarobba