Remove sqlite support

- Changed the database entry in the config to be optional

[#74744188]

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>
Clean up spec database usage

- Update README
- Update rake tasks to allow setting DB_CONNECTION_STRING or DB_CONNECTION

[#74744188]

Signed-off-by: Matthew Sykes <matthew.sykes@gmail.com>