Remove TERM=dumb, which is causing difficult-to-reproduce hangs

Summary:
Ref T7119. Three users have now reported that this causes their systems to hang, so it's looking like the cure is worse than the disease.

Revert this until we can eventually reproduce and understand the hang.

Test Plan: Users running into issues have reported that this resolves things.

Reviewers: btrahan

Reviewed By: btrahan

Subscribers: epriestley

Maniphest Tasks: T7119

Differential Revision: https://secure.phabricator.com/D11644