fix oid casting inconsistency

This patch fixes a minor error message inconsistency. When rejecting
input to the oid type, the following error message is emitted:

nconway=# select 'd'::oid;
ERROR: invalid input syntax for type "oid": "d"

This patch removes the quotes from "oid", to make this error message
consistent with the error messages for rejected input to most other
types.

While I suppose it's possible that some applications might be
examining this error message string, (a) this particular error message
isn't very likely be used for that (b) we have error codes now.

I also took the liberty of removing some long-unused code from
src/backend/utils/adt/numutils.c

Unless anyone objects, I'll apply this within 24 hours.

-Neil