Clean up legacy code (#842)

diff --git a/mobly/controllers/android_device_lib/services/snippet_management_service.py b/mobly/controllers/android_device_lib/services/snippet_management_service.py
index 1eeb338..fae60e2 100644
--- a/mobly/controllers/android_device_lib/services/snippet_management_service.py
+++ b/mobly/controllers/android_device_lib/services/snippet_management_service.py
@@ -18,10 +18,6 @@
 
 MISSING_SNIPPET_CLIENT_MSG = 'No snippet client is registered with name "%s".'
 
-# This config is transient and we will remove it after completing the migration
-# from v1 to v2.
-_CLIENT_V2_CONFIG_KEY = 'use_mobly_snippet_client_v2'
-
 
 class Error(errors.ServiceError):
   """Root error type for snippet management service."""