feat(upgrade): remove useless V6 form config (#666)

to be able to upgrade to Subscription more easily

Only the logics around the config file is removed.
The rest of the support of V6 form is still to be removed.
Related failing tests have been removed. This is dead code, so the risk is known and accepted.

Closes [RUNTIME-127](https://bonitasoft.atlassian.net/browse/RUNTIME-127)