plymouth: remove substituteInPlace to fix build failure (#338)

Remove the 'substituteInPlace' function call because the new SVG no
longer contains the matching patterns.

Closes: #337

Updates the SHA due to changed contents of the SVG file:
NixOS/nixos-artwork#120

modules/plymouth/nixos.nix

@@ -55,13 +55,7 @@ in {
       # the entire repository for a single SVG file.
       default = pkgs.fetchurl {
         url = "https://raw.githubusercontent.com/NixOS/nixos-artwork/f84c13adae08e860a7c3f76ab3a9bef916d276cc/logo/nix-snowflake-colours.svg";
-        # Reduce size
-        postFetch = ''
-          substituteInPlace $out \
-            --replace-fail "141.5919" "70.79595" \
-            --replace-fail "122.80626" "61.40313"
-        '';
-        sha256 = "4+MWdqESKo9omd3q0WfRmnrd3Wpe2feiayMnQlA4izU=";
+        sha256 = "pHYa+d5f6MAaY8xWd3lDjhagS+nvwDL3w7zSsQyqH7A=";
       };
     };