Reenable 'Backfilled events whose prev_events...' sytest (#10292)

Now that we've fixed it.

changelog.d/10292.misc

@@ -0,0 +1 @@
+Reenable a SyTest after it has been fixed.

sytest-blacklist

@@ -45,5 +45,4 @@ Peeked rooms only turn up in the sync for the device who peeked them
 
 # Blacklisted due to changes made in #10272
 Outbound federation will ignore a missing event with bad JSON for room version 6
-Backfilled events whose prev_events are in a different room do not allow cross-room back-pagination
 Federation rejects inbound events where the prev_events cannot be found