Remove an unused function GetWalRcvWriteRecPtr

Hi,

The function GetWalRcvWriteRecPtr isn't being used anywhere, however
pg_atomic_read_u64(&walrcv->writtenUpto); (reading writtenUpto without
spinlock) is being used directly in pg_stat_get_wal_receiver
walreceiver.c. We either make use of the function instead of
pg_atomic_read_u64(&walrcv->writtenUpto); or remove it. Since there's
only one function using walrcv->writtenUpto right now, I prefer to
remove the function to save some LOC (13).

Attaching patch. Thoughts?

Regards,
Bharath Rupireddy.