actor: remove actor propagation backcompat measures
Created by: bobheadxi
Removes backcompat measures introduced in https://github.com/sourcegraph/sourcegraph/pull/28117 and https://github.com/sourcegraph/sourcegraph/pull/28487 to enforce the new standard method of actor propagation from https://github.com/sourcegraph/sourcegraph/issues/27918.
Closes https://github.com/sourcegraph/sourcegraph/issues/28451
Going to merge this after the 3.35 branch cut scheduled for dec 17 (https://github.com/sourcegraph/sourcegraph/issues/28613). This should give plenty of time for all deployments to update, including on-prem instances