remove "requirements" mode from propose_update.sh
remove "requirements" mode from propose_update.sh

We do not automatically propose updates to projects' requirements, so
we don't need this branch of logic any more.

Depends-On: https://review.openstack.org/555402/
Change-Id: I511ead56a6a6efecde25ce6b118256aa0560023a
Signed-off-by: Doug Hellmann <0798ebecfb7bc4dfb9d0445c2fa030eb64e1fa1c@doughellmann.com>

openstack-infra/project-config,openstack-infra/project-config