Revert "safekeeper: check for non-consecutive writes in safekeeper.rs" (#8771)

Reverts neondatabase/neon#8640

This broke `test_last_log_term_switch` via a merge race of some kind.

safekeeper/tests/walproposer_sim/safekeeper_disk.rs

@@ -177,10 +177,6 @@ impl DiskWALStorage {
 
 #[async_trait::async_trait]
 impl wal_storage::Storage for DiskWALStorage {
-    // Last written LSN.
-    fn write_lsn(&self) -> Lsn {
-        self.write_lsn
-    }
     /// LSN of last durably stored WAL record.
     fn flush_lsn(&self) -> Lsn {
         self.flush_record_lsn