Revert "Add FallbackRouter for LLM failover support" (#1105)

Author: xingyaoww

examples/01_standalone_sdk/27_llm_fallback.py

@@ -1,5 +1,4 @@
 from openhands.sdk.llm.router.base import RouterLLM
-from openhands.sdk.llm.router.impl.fallback import FallbackRouter
 from openhands.sdk.llm.router.impl.multimodal import MultimodalRouter
 from openhands.sdk.llm.router.impl.random import RandomRouter
 
@@ -8,5 +7,4 @@
     "RouterLLM",
     "RandomRouter",
     "MultimodalRouter",
-    "FallbackRouter",
 ]