Remove oidValue

This function hasn't been very relevant for a *long* time now,  and
we'll be dropping support for libpq 8.4 real soon.

You can fetch oids using RETURNING since postgresql 8.3,  and the
default has been to create tables without oids for some time before
that.   So while recent libpqs still have oidValue,  and you can
use newer libpqs to connect to older postgresqls,  I still very
much doubt this will impact anybody.

In particular,  none of the reverse dependencies of postgresql-libpq
on hackage make use of oidValue.  If anybody really desperately
needs this function,  then it should be no difficulty to write a
separate binding for it.