These seem to have been used for the builtin documentation before the
 current docstring solution. Currently we do nothing with decorators
 except a few hardcoded ones, and this declaration system is
 needlessly complicated.

This also fixes a bug - decorator declarations were never *removed*
 from a function, so removing a decorator would have no effect until
 renaming or moving the function.

ClassDeclaration{,Data} are now empty boilerplate-only shells, but might
 be useful for something else. Easy to remove later if not.

Tested that the effects of decorators now go away when they're edited out.

Removes the decorator duchaintests, because they don't make sense without decorator declarations. The visible behaviour we actually implement (static/classmethods) is already included in several other tests.

No known regressions.