replication: snapshot cleanup: only attempt to remove snapshots that exist

replication: snapshot cleanup: only attempt to remove snapshots that exist

Since commit a6f5b35 ("replication: prepare: include volumes without
snapshots in the result"), attempts would be made to remove previous
replication snapshots from volumes on which they didn't exist. This
was noticed by Thomas since the output of a replication test in
pve-manager changed.

The issue is not completely new, i.e. there was no check that the
(previous) replication snapshot acutally exists before attempting
removal during the cleanup phase. Fix the issue by adding such a
check.

The $replicate_snapshots hash is only used for this, so the change
there is fine.

Fixes: a6f5b35 ("replication: prepare: include volumes without snapshots in the result")
Reported-by: Thomas Lamprecht <t.lamprecht@proxmox.com>
Signed-off-by: Fiona Ebner <f.ebner@proxmox.com>