Remove rulemakings feature flag from codebase

[pkfec]

Summary

What we're after:
Before, Rulemakings was behind a feature flag to expose the endpoint in lower environments. Now that Rulemakings went live on March 10, we should consider removing the feature flag from the openFEC codebase and also removing this environment variable from the cloud.gov space across all three environments.

Completion criteria

• Remove the FEC_FEATURE_RULEMAKINGS reference in rest.py and api_setup.py
• Rulemakings FEC_FEATURE_RULEMAKINGS is removed from all 3 env