remove "opt_in" parameter from instance, consolidate duplicative managed gRPC test cases
Summary: removed a few places where we were still implying there was a difference between the default flow and the gRPC server flow, now that the managed gRPC server flow is the default flow.
Test Plan: BK + Azure
Reviewers: alangenfeld, sashank, prha
Reviewed By: alangenfeld
Differential Revision: https://dagster.phacility.com/D5167