Revert "Move global user storage from WeakValueDictionary to dict"

This reverts commit cb2363f0fd.

This lead to memory leaks due to insufficient tracking, assuming that
the members intent was enabled.

discord/user.py

@@ -429,29 +429,12 @@ class User(BaseUser, discord.abc.Messageable):
         Specifies if the user is a system user (i.e. represents Discord officially).
     """
 
-    __slots__ = ('_stored',)
-
-    def __init__(self, *, state: ConnectionState, data: UserPayload) -> None:
-        super().__init__(state=state, data=data)
-        self._stored: bool = False
+    __slots__ = ('__weakref__',)
 
     def __repr__(self) -> str:
         return f'<User id={self.id} name={self.name!r} discriminator={self.discriminator!r} bot={self.bot}>'
 
-    def __del__(self) -> None:
-        try:
-            if self._stored:
-                self._state.deref_user(self.id)
-        except Exception:
-            pass
-
-    @classmethod
-    def _copy(cls, user: User):
-        self = super()._copy(user)
-        self._stored = False
-        return self
-
-    async def _get_channel(self) -> DMChannel:
+    async def _get_channel(self):
         ch = await self.create_dm()
         return ch