refactor: remove feature flag logic for larger dyno sizes

[k80bowman]

Since larger dyno sizes are now available in production, we are removing the logic from ps:scale and ps:type that checks for the feature flag.

Testing

• Pull down this branch and run yarn && yarn build
• Use a test app with at least one dyno process
• Run ./bin/run ps:type -a APP_NAME to see what type of dyno is currently running
• Run ./bin/run ps:type performance-2xl -a APP_NAME to change to a performance-2xl dyno
• Run ./bin/run ps:scale web=2:performance-2xl -a APP_NAME to scale up to 2 performance-2xl dynos
• Run ./bin/run ps:type -a APP_NAME to validate that your app is now using 2 performance-2xl dynos
• Run ./bin/run ps:scale web=1:basic -a APP_NAME to return your app to running 1 basic dyno (or substitute whatever your original configuration was)

Resolves internal work item

[eablack]

Looks great! I think we can also remove

https://github.com/heroku/cli/pull/2985/files#diff-3ee81dacbf49bbbf49b72b8aabaa0f1f19fad5da2bc612afc919efa8f829d31dL11-L17

and

https://github.com/heroku/cli/pull/2985/files#diff-17016820d6177fdc6dc5e3ec42c5498adbdfc4f41284a87f45e59a45f7d4f060L11-L17