stop writing metadata file (#6769)

Building atop #6777, this PR removes the code that writes the `metadata`
file and adds a piece of migration code that removes any remaining
`metadata` files.

We'll remove the migration code after this PR has been deployed.

part of https://github.com/neondatabase/neon/issues/6663

More cleanups punted into follow-up issue, as they touch a lot of code: 
https://github.com/neondatabase/neon/issues/6890

test_runner/regress/test_remote_storage.py

@@ -694,10 +694,8 @@ def test_empty_branch_remote_storage_upload_on_restart(neon_env_builder: NeonEnv
     # index upload is now hitting the failpoint, it should block the shutdown
     env.pageserver.stop(immediate=True)
 
-    local_metadata = (
-        env.pageserver.timeline_dir(env.initial_tenant, new_branch_timeline_id) / "metadata"
-    )
-    assert local_metadata.is_file()
+    timeline_dir = env.pageserver.timeline_dir(env.initial_tenant, new_branch_timeline_id)
+    assert timeline_dir.is_dir()
 
     assert isinstance(env.pageserver_remote_storage, LocalFsStorage)
 

test_runner/regress/test_tenant_delete.py

@@ -130,7 +130,6 @@ FAILPOINTS = [
     "timeline-delete-before-index-deleted-at",
     "timeline-delete-before-rm",
     "timeline-delete-before-index-delete",
-    "timeline-delete-after-rm-dir",
 ]
 
 FAILPOINTS_BEFORE_BACKGROUND = [

test_runner/regress/test_tenant_relocation.py

@@ -157,10 +157,7 @@ def switch_pg_to_new_pageserver(
     timeline_to_detach_local_path = origin_ps.timeline_dir(tenant_id, timeline_id)
     files_before_detach = os.listdir(timeline_to_detach_local_path)
     assert (
-        "metadata" in files_before_detach
-    ), f"Regular timeline {timeline_to_detach_local_path} should have the metadata file, but got: {files_before_detach}"
-    assert (
-        len(files_before_detach) >= 2
+        len(files_before_detach) >= 1
     ), f"Regular timeline {timeline_to_detach_local_path} should have at least one layer file, but got {files_before_detach}"
 
     return timeline_to_detach_local_path

test_runner/regress/test_timeline_delete.py

@@ -136,12 +136,9 @@ DELETE_FAILPOINTS = [
     "timeline-delete-before-index-deleted-at",
     "timeline-delete-before-schedule",
     "timeline-delete-before-rm",
-    "timeline-delete-during-rm",
     "timeline-delete-after-rm",
     "timeline-delete-before-index-delete",
     "timeline-delete-after-index-delete",
-    "timeline-delete-after-rm-metadata",
-    "timeline-delete-after-rm-dir",
 ]
 
 
@@ -801,7 +798,7 @@ def test_timeline_delete_resumed_on_attach(
         )
 
     # failpoint before we remove index_part from s3
-    failpoint = "timeline-delete-during-rm"
+    failpoint = "timeline-delete-after-rm"
     ps_http.configure_failpoints((failpoint, "return"))
 
     env.pageserver.allowed_errors.extend(