boost: use default toolset option

for 1.66 & 1.67 we were overriding the default toolset option. This
meant that gcc-cross was not being selected properly, breaking the
cross toolchain.

/cc @dtzWill

pkgs/development/libraries/boost/1.66.nix

@@ -8,6 +8,4 @@ callPackage ./generic.nix (args // rec {
     # SHA256 from http://www.boost.org/users/history/version_1_66_0.html
     sha256 = "5721818253e6a0989583192f96782c4a98eb6204965316df9f5ad75819225ca9";
   };
-
-  toolset = if stdenv.cc.isClang then "clang" else null;
 })

pkgs/development/libraries/boost/1.67.nix

@@ -8,6 +8,4 @@ callPackage ./generic.nix (args // rec {
     # SHA256 from http://www.boost.org/users/history/version_1_66_0.html
     sha256 = "2684c972994ee57fc5632e03bf044746f6eb45d4920c343937a465fd67a5adba";
   };
-
-  toolset = if stdenv.cc.isClang then "clang" else null;
 })