gccNGPackages_15.gcc: drop unused libelf dependency

This mirrors the https://github.com/NixOS/nixpkgs/pull/187234 change
where `libelf` was removed from `gcc` packages. `libelf` is not used
by `gcc` since `4.6.0` version.

pkgs/development/compilers/gcc/ng/common/gcc/default.nix

@@ -23,7 +23,6 @@
   gmp,
   libmpc,
   mpfr,
-  libelf,
   perl,
   texinfo,
   which,
@@ -78,7 +77,6 @@ stdenv.mkDerivation (finalAttrs: {
       gmp
       libmpc
       mpfr
-      libelf
     ]
     ++ lib.optional (isl != null) isl
     ++ lib.optional (zlib != null) zlib;