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/src/safekeeper.rs

@@ -875,29 +875,6 @@ where
             return Ok(Some(AcceptorProposerMessage::AppendResponse(resp)));
         }
 
-        // Disallow any non-sequential writes, which can result in gaps or
-        // overwrites. If we need to move the pointer, ProposerElected message
-        // should have truncated WAL first accordingly. Note that the first
-        // condition (WAL rewrite) is quite expected in real world; it happens
-        // when walproposer reconnects to safekeeper and writes some more data
-        // while first connection still gets some packets later. It might be
-        // better to not log this as error! above.
-        let write_lsn = self.wal_store.write_lsn();
-        if write_lsn > msg.h.begin_lsn {
-            bail!(
-                "append request rewrites WAL written before, write_lsn={}, msg lsn={}",
-                write_lsn,
-                msg.h.begin_lsn
-            );
-        }
-        if write_lsn < msg.h.begin_lsn && write_lsn != Lsn(0) {
-            bail!(
-                "append request creates gap in written WAL, write_lsn={}, msg lsn={}",
-                write_lsn,
-                msg.h.begin_lsn,
-            );
-        }
-
         // Now we know that we are in the same term as the proposer,
         // processing the message.
 
@@ -1028,10 +1005,6 @@ mod tests {
 
     #[async_trait::async_trait]
     impl wal_storage::Storage for DummyWalStore {
-        fn write_lsn(&self) -> Lsn {
-            self.lsn
-        }
-
         fn flush_lsn(&self) -> Lsn {
             self.lsn
         }

safekeeper/src/wal_storage.rs

@@ -37,8 +37,6 @@ use utils::{id::TenantTimelineId, lsn::Lsn};
 
 #[async_trait::async_trait]
 pub trait Storage {
-    // Last written LSN.
-    fn write_lsn(&self) -> Lsn;
     /// LSN of last durably stored WAL record.
     fn flush_lsn(&self) -> Lsn;
 
@@ -329,10 +327,6 @@ impl PhysicalStorage {
 
 #[async_trait::async_trait]
 impl Storage for PhysicalStorage {
-    // Last written LSN.
-    fn write_lsn(&self) -> Lsn {
-        self.write_lsn
-    }
     /// flush_lsn returns LSN of last durably stored WAL record.
     fn flush_lsn(&self) -> Lsn {
         self.flush_record_lsn

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