Remove all the is_[A-Z].* from global namespace. They have been deprecated for over two years.

step 1) remove the imports of is_* from rings/all.py (#15076)

step 2) remove the imports of is_* from matrix/all.py (#15098)

step 3) idem for modules/all.py and structure/all.py (#15333)

step 4) remove otherwise unused (i.e., not re-imported) imports of is_[A-Z]* (this ticket)

step 5) remove the remaining imports of is_[A-Z]* (this ticket)

Depends on #15076
Depends on #15098
Depends on #15333

Component: performance

Author: Marc Mezzarobba

Branch/Commit: b948874

Reviewer: Peter Bruin

Issue created by migration from https://trac.sagemath.org/ticket/14329