Remove useless code from CreateReplicationSlot.

Remove useless code from CreateReplicationSlot.

According to the comments, we initialize sendTimeLineIsHistoric
and sendTimeLine here for the benefit of WalSndSegmentOpen.
However, the only way that can happen is if logical_read_xlog_page
calls WALRead. And since logical_read_xlog_page initializes the
same global variables internally, we don't need to also do it here.

These initializations have been here since replication slots were
introduced in commit 858ec11858a914d4c380971985709b6d6b7dd6fc. They
were certainly useless at that time, too, because logical decoding
didn't yet exist then, and physical replication doesn't examine any
WAL at the time of slot creation. I haven't checked all the
intermediate versions, but I suspect there's no point at which
this code ever did anything useful.

To reduce future confusion, remove the code. Since there's no
functional defect, no back-patch.

Discussion: http://postgr.es/m/CA+TgmobSWzacEs+r6C-7DrOPDHoDar4i9gzxB3SCBr5qjnLmVQ@mail.gmail.com