Remove unused function GetOldestWALSendPointer from walsender code.

That unused function was introduced as a sample because synchronous
replication or replication monitoring tools might need it in the future.
Recently commit 989be081 added the function SyncRepGetOldestSyncRecPtr
which provides almost the same functionality for multiple synchronous
standbys feature. So it's time to remove that unused sample function.
This commit does that.