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/development/libraries/libjson-rpc-cpp/default.nix

@@ -1,5 +1,5 @@
 { stdenv, fetchFromGitHub, cmake, jsoncpp, argtable, curl, libmicrohttpd
-, doxygen, catch, pkgconfig, git, gcc6
+, doxygen, catch, pkgconfig, git
 }:
 
 stdenv.mkDerivation rec {
@@ -39,7 +39,7 @@ stdenv.mkDerivation rec {
 
     function fixRunPath {
       p=$(patchelf --print-rpath $1)
-      q="$p:${stdenv.lib.makeLibraryPath [ gcc6 jsoncpp argtable libmicrohttpd curl ]}:$out/lib"
+      q="$p:${stdenv.lib.makeLibraryPath [ jsoncpp argtable libmicrohttpd curl ]}:$out/lib"
       patchelf --set-rpath $q $1
     }
 
@@ -53,7 +53,7 @@ stdenv.mkDerivation rec {
     cp -r Install/* $out
   '';
 
-  nativeBuildInputs = [ pkgconfig gcc6 ];
+  nativeBuildInputs = [ pkgconfig ];
   buildInputs = [ cmake jsoncpp argtable curl libmicrohttpd doxygen catch ];
 
   meta = with stdenv.lib; {