[dagit] Remove session-token header

Summary: Just use the value from the cookie -- we don't need the extra header after all.

Test Plan: Validated with dgibson. Commented out the code that set the session token header, and auth worked as desired.

Reviewers: dgibson, sidkmenon, sashank, max

Reviewed By: sidkmenon

Differential Revision: https://dagster.phacility.com/D8469