Remove permission_allowed hook entirely, refs #2528

The permission_allowed hook has been fully replaced by permission_resources_sql.
This commit removes:
- hookspec definition from hookspecs.py
- 4 implementations from default_permissions.py
- implementations from test plugins (my_plugin.py, my_plugin_2.py)
- hook monitoring infrastructure from conftest.py
- references from fixtures.py
- Also fixes test_get_permission to use ds.get_action() instead of ds.get_permission()
- Removes 5th column (source_plugin) from PermissionSQL queries

This completes the migration to the SQL-based permission system.

tests/plugins/my_plugin_2.py

@@ -113,24 +113,6 @@ def actor_from_request(datasette, request):
     return inner
 
 
-@hookimpl
-def permission_allowed(datasette, actor, action):
-    # Testing asyncio version of permission_allowed
-    async def inner():
-        assert (
-            2
-            == (
-                await datasette.get_internal_database().execute("select 1 + 1")
-            ).first()[0]
-        )
-        if action == "this_is_allowed_async":
-            return True
-        elif action == "this_is_denied_async":
-            return False
-
-    return inner
-
-
 @hookimpl
 def prepare_jinja2_environment(env, datasette):
     env.filters["format_numeric"] = lambda s: f"{float(s):,.0f}"