Remove pluggable global connectors

We only have one connector (the system connector), so get rid of the abstraction.
Also, since the system needs to have specialized logic for dealing with sys tables,
get rid of the system connector. We'll revisit this once we figure out how we
want to model it going forward.
Don't forward requests for well-known schemas to connector

This requires hard-coding knowledge about sys and information_schema in
MetadataManager. It's a hack until we figure out how we want to model
these going forward.