Remove is_production flag from Sites header/index

[braddf]

Describe the bug

Unnecessary code relating to NEXT_PUBLIC_IS_PRODUCTION env var on the Sites header link and the main view components in nowcasting-app/pages/index.tsx.

To Reproduce

Steps to reproduce the behavior:

1. Go to Quartz UI
2. Sites Header link is greyed out if NEXT_PUBLIC_IS_PRODUCTION is set to true

Expected behavior

The flag may be useful for anything that should look the same on Production and Staging for testing, but not on all Preview branches (differences in Dev/Prod API etc.) so we will keep it, but just don't need these particular disabled elements tied to it anymore.