Remove feature toggle around newsletter subscription form.

Reverts the following commits:

- "Add feature toggle around newsletter subscription form." 9d5b5c6c19baa6c625036783857e2156122644bc
- "Use background step to check for the state of the feature." a1c32333e6fd7dbf5eed240df22968f1f88e74c6
- "Run the newsletter subscription acceptance tests given the feature is enabled." 0b5a7a06322a05cfbda13f5d5562f5c3d1e5b566
- "Little trick to hide the grey line if the newsletter subscription form is hidden." 1055982460009bc2933755b30e6ad39d3fcdc6ec

@ALiaume Newsletter signup feature ready to go live.