Get rid of Pretty interface in favor of ToExpr

[lehins]

Defining custom pretty instances is time consuming and frustrating, moreover it has no value as generically derived ToExpr instances provide us with superior functionality: they are derivable and they provide diffing out of the box.
Most importantly, however, they are unreadable and error prone, due to requiring manual definition.

Test.Cardano.Ledger.Generic.PrettyCore module and related functionality needs to go!