Remove SYNAPSE_USE_FROZEN_DICTS environment variable (#18123)

I got rid of the `SYNAPSE_USE_FROZEN_DICTS` environment variable because
it will be overridden by the Synapse worker apps anyway and if we want
to support `SYNAPSE_USE_FROZEN_DICTS`, it should be in
`synapse/config/server.py`. It's also not documented so I'm assuming no
one is using it anyway.

Spawning from looking at the frozen dict stuff during the review of
#18103 (comment)

Author: MadLittleMods

changelog.d/18123.misc

@@ -0,0 +1 @@
+Remove undocumented `SYNAPSE_USE_FROZEN_DICTS` environment variable.

synapse/events/__init__.py

@@ -22,7 +22,6 @@
 
 import abc
 import collections.abc
-import os
 from typing import (
     TYPE_CHECKING,
     Any,
@@ -48,21 +47,21 @@
 from synapse.types import JsonDict, StrCollection
 from synapse.util.caches import intern_dict
 from synapse.util.frozenutils import freeze
-from synapse.util.stringutils import strtobool
 
 if TYPE_CHECKING:
     from synapse.events.builder import EventBuilder
 
-# Whether we should use frozen_dict in FrozenEvent. Using frozen_dicts prevents
-# bugs where we accidentally share e.g. signature dicts. However, converting a
-# dict to frozen_dicts is expensive.
-#
-# NOTE: This is overridden by the configuration by the Synapse worker apps, but
-# for the sake of tests, it is set here while it cannot be configured on the
-# homeserver object itself.
 
-USE_FROZEN_DICTS = strtobool(os.environ.get("SYNAPSE_USE_FROZEN_DICTS", "0"))
+USE_FROZEN_DICTS = False
+"""
+Whether we should use frozen_dict in FrozenEvent. Using frozen_dicts prevents
+bugs where we accidentally share e.g. signature dicts. However, converting a
+dict to frozen_dicts is expensive.
 
+NOTE: This is overridden by the configuration by the Synapse worker apps, but
+for the sake of tests, it is set here because it cannot be configured on the
+homeserver object itself.
+"""
 
 T = TypeVar("T")