bug#61730: 30.0.50; Compiler warnings for delq and delete

[Mattias Engdegård]

On master there is now a function declaration and property, 
`important-return-value`, that is used to mark functions that the compiler 
should warn about when called without their value being used.

I haven't set the property on delq and delete yet; about half of the cases seem 
to be false positives but it may be worth the effort to clean up those too.
I did fix a correct but somewhat daft use of cl-delete in python.el and enabled 
the warning for that function.