wavpack: clean up obsolete copy

Now that autoreconf runs autopoint, this is done for us automatically.

pkgs/by-name/wa/wavpack/package.nix

···
   ];
   buildInputs = [ libiconv ];
 
-  # autogen.sh:9
-  preAutoreconf = "cp ${gettext}/share/gettext/config.rpath .";
-
   src = fetchFromGitHub {
     owner = "dbry";
     repo = "WavPack";