Remove PointerIsValid()

Remove PointerIsValid()

This doesn't provide any value over the standard style of checking the
pointer directly or comparing against NULL.

Also remove related:
- AllocPointerIsValid() [unused]
- IndexScanIsValid() [had one user]
- HeapScanIsValid() [unused]
- InvalidRelation [unused]

Leaving HeapTupleIsValid(), ItemIdIsValid(), PortalIsValid(),
RelationIsValid for now, to reduce code churn.

Reviewed-by: Jacob Champion <jacob.champion@enterprisedb.com>
Discussion: https://www.postgresql.org/message-id/flat/ad50ab6b-6f74-4603-b099-1cd6382fb13d%40eisentraut.org
Discussion: https://www.postgresql.org/message-id/CA+hUKG+NFKnr=K4oybwDvT35dW=VAjAAfiuLxp+5JeZSOV3nBg@mail.gmail.com
Discussion: https://www.postgresql.org/message-id/bccf2803-5252-47c2-9ff0-340502d5bd1c@iki.fi