Remove unused function from walsender.c

Hi,

There is an unused function GetOldestWALSendPointer() in walsender.c.
Per comment, it was introduced because we may need it in the future for
synchronous replication.

Now we have very similar function SyncRepGetOldestSyncRecPtr() in
syncrep.c. Which makes me think that GetOldestWALSendPointer()
no longer needs to be maintained. So, is it time to remove that unused
function?

Regards,

-- 
Fujii Masao