Collapse PG default extractoin of most types to single regex

For any type that is represented as a string and then type cast, we do
not need separate regular expressions for the various types. No function
will match this regex. User defined types *should* match this, so that
the type object can decide what to do with the value.
Merge pull request #15504 from sgrif/sg-postgres-defaults

Collapse PG default extractoin of most types to single regex