The copy/out/read functions for it were wrong: a Bitmapset is not a Node,
so one should use e.g. COPY_BITMAPSET_FIELD() instead of COPY_NODE_FIELD()
for them. But since the fields are currently unused, let's just remove them.

These fields will be resurrected soon, by the PostgreSQL 8.3 merge, as they
were introduced in PostgreSQL 8.3. Then they will actually be used, too.