Remove unused file: list-db-types

[alxndrsn]

This was previously referenced in README.md:

node-postgres/README.md

Line 46 in 32c4934

fun looking up the [oids for the datatypes in your bound queries](http://github.com/brianc/node-postgres/blob/master/script/list-db-types.js)

, but this was removed in 827e7d7.

[brianc]

👍 ty!