Remove homepage-feature-slider feature flag code

[corywatilo]

Spotted in plans.js:

                    <TrackedCTA
                        to="https://app.posthog.com/signup"
                        className="my-7"
                        event={{ name: 'select edition: clicked get started', type: 'cloud' }}
                        featureFlag={{ response: 'slider_on', name: 'homepage-feature-slider' }}
                    >
                        Get started
                    </TrackedCTA>