[dagit] Remove materialization timestamp from Assets page
Summary: Remove the materialization timestamp from the /assets list query. It appears to be pretty slow, and it's unclear whether we need it right now.
Test Plan: View /assets, verify that the list renders without the removed column.
Reviewers: prha, sandyryza, schrockn
Reviewed By: schrockn
Differential Revision: https://dagster.phacility.com/D4617