De-implement non-working / testing copy_internals from dml

De-implement non-working / testing copy_internals from dml

The DML constructs will need to have traverse_internals
symbols set up; as there are currently non-working /non-used
_copy_internals methods, just remove these for now as they
are unlikely to be working correctly in any case.

Additionally remove an errant "return" statement noted
on the same issue from the MySQL dialect.

Fixes: #5060
Change-Id: I289005af04192e4c755d53244b1ea0711c266c6c