treewide: purge explicit usage of gcc6

It's now the default. /cc #19456
This makes a real build simplification, because in our current
bootstrapping+aliases, `gcc6` attribute is not the default compiler
but a derivation *built by* the default compiler.

nix-exec didn't build before this commit already

pkgs/applications/graphics/freepv/default.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchurl, libjpeg, mesa, freeglut, zlib, cmake, libX11, libxml2, libpng,
-  libXxf86vm, gcc6 }:
+  libXxf86vm }:
 
 stdenv.mkDerivation {
   name = "freepv-0.3.0";
@@ -10,7 +10,7 @@ stdenv.mkDerivation {
   };
 
   buildInputs = [ libjpeg mesa freeglut zlib cmake libX11 libxml2 libpng
-    libXxf86vm gcc6 ];
+    libXxf86vm ];
 
   postPatch = ''
     sed -i -e '/GECKO/d' CMakeLists.txt