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/ctl/src/draw_timeline_dir.rs

@@ -52,7 +52,6 @@
 
 use anyhow::{Context, Result};
 use pageserver::repository::Key;
-use pageserver::METADATA_FILE_NAME;
 use std::cmp::Ordering;
 use std::io::{self, BufRead};
 use std::path::PathBuf;
@@ -159,10 +158,6 @@ pub fn main() -> Result<()> {
         let line = PathBuf::from_str(&line).unwrap();
         let filename = line.file_name().unwrap();
         let filename = filename.to_str().unwrap();
-        if filename == METADATA_FILE_NAME {
-            // Don't try and parse "metadata" like a key-lsn range
-            continue;
-        }
         let (key_range, lsn_range) = parse_filename(filename);
         files.push(Layer {
             filename: filename.to_owned(),