Remove excess assert from InvalidatePossiblyObsoleteSlot()

Remove excess assert from InvalidatePossiblyObsoleteSlot()

ca307d5cec90 introduced keeping WAL segments by slot's last saved restart LSN.
It also added an assertion that the slot's restart LSN never goes backward.
However, situations when the restart LSN goes backward have been spotted by
buildfarm animals and investigated in the thread.

When pg_receivewal starts the replication, it sets the last replayed LSN to
the beginning of the segment, which is older than what
ReplicationSlotReserveWal() set for the slot.  A similar situation can happen
to pg_basebackup.  When standby reconnects to the primary, it sends the last
replayed LSN, which might be older than the last confirmed flush LSN.  In
both these situations, a concurrent checkpoint may trigger an assert trap.

Based on ideas from Vitaly Davydov <v.davydov@postgrespro.ru>,
Hayato Kuroda (Fujitsu) <kuroda.hayato@fujitsu.com>,
Vignesh C <vignesh21@gmail.com>,
Amit Kapila <amit.kapila16@gmail.com>.

Reported-by: Vignesh C <vignesh21@gmail.com>
Reported-by: Tom Lane <tgl@sss.pgh.pa.us>
Discussion: http://postgr.es/m/CALDaNm3s-jpQTe1MshsvQ8GO%3DTLj233JCdkQ7uZ6pwqRVpxAdw%40mail.gmail.com
Reviewed-by: Vignesh C <vignesh21@gmail.com>
Reviewed-by: Amit Kapila <amit.kapila16@gmail.com>