pg: remove the useless connection string

Hi Sequelize !

When trying to connect to node-postgresql, we don't have to give a connection string, we can also give directly the connection object.

This has the following benefices : 
- no useless string generation then string parsing
- allow us to put smarter data in the configuration object. For example, this allow the connection to a pg database through a named socket via `{host: '/path/to/socket/directory'}`

I didn't see any test to change. It works on our machines with our configuration.