Fix import of the default SAML mapping provider. (#10477)

Fix a circular import, which was causing exceptions on boot if SAML
was configured.

changelog.d/10477.bugfix

+Fix bug introduced in Synapse 1.38 which caused an exception at startup when SAML authentication was enabled.

synapse/handlers/_base.py

@@ -15,8 +15,6 @@
 import logging
 from typing import TYPE_CHECKING, Optional
 
-import synapse.state
-import synapse.storage
 import synapse.types
 from synapse.api.constants import EventTypes, Membership
 from synapse.api.ratelimiting import Ratelimiter