api/server: Drop dead code

It was a class parameter from the swh.core.api.asynchronous.RPCServerApp which got
removed some time ago.

This is not defined in the swh.core.api.RPCServerApp.

swh/vault/api/server.py

@@ -33,7 +33,6 @@ def get_vault():
 
 
 class VaultServerApp(RPCServerApp):
-    client_exception_classes = (NotFoundExc,)
     extra_type_decoders = DECODERS
     extra_type_encoders = ENCODERS