repo-updater: Remove feature flag to enable / disable repo update jitter

Created by: indradhanush

Reverts sourcegraph/sourcegraph#30154

Part of investigation of #30110.

We had set the feature flag to verify if the jitter was the root cause of the alert 0+ rate of growth of update queue length over 5 minutes to get triggered but after ~2 weeks in production with jitter disabled, it seems like this is unrelated. Here's the graph for the related metric since the time we disabled the jitter feature.

Link to the dashboard.