linuxPackages.lttng-modules: Drop unneeded NIX_CFLAGS_COMPILE

Added wayyyy back in 2676cf9525, this
NIX_CFLAGS_COMPILE usage is now unneeded.

It *necessarily* had to be unneeded, as using kernelModuleMakeFlags and
the kernel's out-of-tree build tooling forces the unwrapped `cc` to be
used.

See here: https://github.com/NixOS/nixpkgs/blame/f86e9fe549f551d6bb738a1d01efa7d61b2da3e7/pkgs/os-specific/linux/kernel/common-flags.nix#L10-L11

pkgs/os-specific/linux/lttng-modules/default.nix

@@ -21,8 +21,6 @@ stdenv.mkDerivation rec {
 
   hardeningDisable = [ "pic" ];
 
-  env.NIX_CFLAGS_COMPILE = "-Wno-error=implicit-function-declaration";
-
   makeFlags = kernelModuleMakeFlags ++ [
     "KERNELDIR=${kernel.dev}/lib/modules/${kernel.modDirVersion}/build"
     "INSTALL_MOD_PATH=${placeholder "out"}"