[easy] remove warning in grpc handle del
Summary: This was added with the best of intentions but it just causing problems / periodic logspew when you quit dagit. Seems like destructors in python are best avoided.
Test Plan: BK, no more logspew when you ctrl-C dagit process
Reviewers: alangenfeld, sashank, prha, schrockn
Reviewed By: prha
Differential Revision: https://dagster.phacility.com/D5178