Drop rds parameter groups as part of db upgrade

[BryanQuigley]

The way we specify db parameter group doesn't work well for upgrades. Removing it is much simpler.

Notes

Happy to update and remove terraform-rds entirely as I've done for other projects - this may help with eventually upgrading terraform.

Testing Instructions

• Confirm stg still works
• Review parameter group values

Resolves #859

[BryanQuigley]

No real changes - just to rekick Jenkins build which is finally happy.