Make mode required in environment class construction
This was written in a time in place where mode was optional. Now if the user does not specify a mode in a pipeline
one is created on their behalf. Take advantage of the new modeful world to eliminate a dead code path.
Test Plan: BK
Reviewers: alangenfeld, max, themissinghlink
Reviewed By: themissinghlink
Differential Revision: https://dagster.phacility.com/D1487