Template Parts: Remove forced Spotlight Mode on selection

[jameskoster]

We've tried a few experiments in the Site Editor to clarify the boundaries of Template Parts. There are currently three hints:

1. The template part name is displayed in the top bar when it – or any of its child blocks – is selected
2. The template part is outlined, both on hover (which helps with selection) and on click
3. Blocks outside the selected template part are semi-transparent, similar to the Editor's 'Spotlight mode'

Three hints is perhaps a bit excessive. I propose we remove the third one for two reasons:

1. It's easy to add back via user preference – just enable Spotlight mode to achieve a similar result
2. It's essentially doing the same job as the first hint.