treewide: get rid of invalid buildPhases argument

I don't know where this comes from (I accidentally did that as well
once), but some derivations seem to use `buildPhases` rather than
`phases` in their derivations.

This kills all improper usages as the lack of a `phases` argument
didn't break the build, so this can be safely removed.

pkgs/misc/screensavers/i3lock-pixeled/default.nix

@@ -16,7 +16,6 @@ stdenv.mkDerivation rec {
     playerctl
   ];
 
-  buildPhases = [ "unpackPhase" "patchPhase" "installPhase" ];
   makeFlags = [
     "PREFIX=$(out)/bin"
   ];