Remove final declaration for @Child fields.

From the FAQ:

To help partial evaluation, should I declare as many fields as possible as final?
Yes; make fields and parameters final whenever possible, except for fields marked @Child, which must not be final. (This hampers AST rewriting.) Fields annotated with @Children, which hold arrays or other structures, must be final.
Java 8

Also `mvn deploy` no longer works, for no apparent reason. Don't fret, though, luckily `mvn install` seems to do what we want (create the überjar).