firefox: remove superfluous NIX_CFLAGS_COMPILE

It was added for nspr and nss back in the 55.0.3 to 56.0 upgrade. It
also served as a workaround for an undeclared gio-unix-2.0 dependency.
Sometime afterwards nspr was removed, leaving just the two. Since then,
upstream has added a declaration for gio-unix-2.0 (in FF62). As for the
nss include it seemingly has no purpose since current firefox builds
with it removed.

pkgs/applications/networking/browsers/firefox/common.nix

@@ -186,11 +186,6 @@ buildStdenv.mkDerivation ({
     -rpath ${llvmPackages.libunwind.out}/lib
   '';
 
-  NIX_CFLAGS_COMPILE = toString [
-    "-I${glib.dev}/include/gio-unix-2.0"
-    "-I${nss_pkg.dev}/include/nss"
-  ];
-
   MACH_USE_SYSTEM_PYTHON = "1";
 
   postPatch = ''