Similar to how Apple let people remove built-in iOS apps starting with iOS 10, Apple will let people delete built-in watch apps with watchOS 6 (via TechCrunch).
Built-In Watch Apps
Apple’s built-in watch apps include Alarm, Timber, Stopwatch, Remote, Camera Remote, Radio, ECG, Breathe, and watchOS 6’s Noise and Cycle Tracking. Uninstallation is similar to iOS: Tap and hold on an app icon, wait for it to wiggle, then tap the X that appears.
Or, you can open the Apple Watch app on iOS, into the app settings, and turn off the toggle for “Show on Apple Watch.” Some apps can’t be removed though, like Messages and Heart Rate.
TechCrunch notes that this isn’t available in the current watchOS 6 beta, but sources familiar with the plan say it’s coming in the future.