treewide: get rid of platforms.allBut

Negative reasoning like `allBut` is a bad idea with an open world of
platforms. Concretely, if we add a new, quite different sort of
platform, existing packages with `allBut` will claim they work on it
even though they probably won't.

pkgs/development/libraries/rocksdb/default.nix

@@ -73,7 +73,7 @@ stdenv.mkDerivation rec {
     homepage = http://rocksdb.org;
     description = "A library that provides an embeddable, persistent key-value store for fast storage";
     license = licenses.bsd3;
-    platforms = platforms.allBut [ "i686-linux" ];
+    platforms = platforms.x86_64 ++ platforms.aarch64;
     maintainers = with maintainers; [ adev wkennington ];
   };
 }