pageserver: remove Option<> around remote storage, clean up metadata file refs (#7752)

## Problem

This is historical baggage from when the pageserver could be run with
local disk only: we had a bunch of places where we had to treat remote
storage as optional.

Closes: https://github.com/neondatabase/neon/issues/6890

## Changes

- Remove Option<> around remote storage (in
https://github.com/neondatabase/neon/pull/7722 we made remote storage
clearly mandatory)
- Remove code for deleting old metadata files: they're all gone now.
- Remove other references to metadata files when loading directories, as
none exist.

I checked last 14 days of logs for "found legacy metadata", there are no
instances.

pageserver/src/deletion_queue.rs

@@ -632,7 +632,7 @@ impl DeletionQueue {
     ///
     /// If remote_storage is None, then the returned workers will also be None.
     pub fn new<C>(
-        remote_storage: Option<GenericRemoteStorage>,
+        remote_storage: GenericRemoteStorage,
         control_plane_client: Option<C>,
         conf: &'static PageServerConf,
     ) -> (Self, Option<DeletionQueueWorkers<C>>)
@@ -658,23 +658,6 @@ impl DeletionQueue {
         // longer to flush after Tenants have all been torn down.
         let cancel = CancellationToken::new();
 
-        let remote_storage = match remote_storage {
-            None => {
-                return (
-                    Self {
-                        client: DeletionQueueClient {
-                            tx,
-                            executor_tx,
-                            lsn_table: lsn_table.clone(),
-                        },
-                        cancel,
-                    },
-                    None,
-                )
-            }
-            Some(r) => r,
-        };
-
         (
             Self {
                 client: DeletionQueueClient {
@@ -765,7 +748,7 @@ mod test {
         /// Simulate a pageserver restart by destroying and recreating the deletion queue
         async fn restart(&mut self) {
             let (deletion_queue, workers) = DeletionQueue::new(
-                Some(self.storage.clone()),
+                self.storage.clone(),
                 Some(self.mock_control_plane.clone()),
                 self.harness.conf,
             );
@@ -875,7 +858,7 @@ mod test {
         let mock_control_plane = MockControlPlane::new();
 
         let (deletion_queue, worker) = DeletionQueue::new(
-            Some(storage.clone()),
+            storage.clone(),
             Some(mock_control_plane.clone()),
             harness.conf,
         );