Remove Unnecessary Uses of `rec`

[sivizius]

This PR removes unnecessary usage of rec. This is not about rec sometimes considered an anti-pattern, this is about code, where this keyword can simply be removed. Because linters like statix complain, they have to configured to ignore sources.nix, which is somewhat annoying. This PR should fix that.

[nmattia]

EDIT: done

There's a test failure because Niv keeps track of the hashes of the different sources.nix versions:

 sources.nix
    has latest version:                   FAIL
      src/Niv/Sources/Test.hs:13:
      expected: "26ed55356db7673935329210a4f8c4a5"
       but got: "a8751de841ac5e0a60f4c2db7e8bbade"

if you want to have a go at it you can add a V29 in src/Niv/Sources.hs with hash "a87...", otherwise I'll have a look this evening!

[sivizius]

I do not understand the test failure or how this is related to my and your changes. Is there something I could do to finish this PR?

[nmattia]

It's unrelated. Thanks for the PR!