Remove individuals from copyright in favor of "nixfmt contributors"

[infinisil]

As discussed in #169

While this does change the copyright notice, which is technically not allowed by MPL-2.0

nixfmt/LICENSE

Lines 198 to 204 in b84dba1

	3.4. Notices
	You may not remove or alter the substance of any license notices
	(including copyright notices, patent notices, disclaimers of warranty,
	or limitations of liability) contained within the Source Code Form of
	the Covered Software, except that You may alter any license notices to
	the extent required to remedy known factual inaccuracies.

I believe it should be fine if we can get all of the individuals approval. @Lucus16 already indicated approval in #169 (comment):

In my case I don't mind you removing my copyright notices

I'm also fine with my name being removed.

We only need @piegamesde's explicit approval now.

Note that this doesn't remove the copyright of the contributions, it just changes the copyright note.

CC @Sereja313 @gromakovsky