csfml: mark linux only

While it's possible to build on darwin, we never had a successful build.

    Undefined symbols for architecture x86_64:
      "sf::microseconds(long long)", referenced from:
          _sfSleep in Sleep.cpp.o

/cc ZHF #45961

pkgs/development/libraries/csfml/default.nix

@@ -25,7 +25,6 @@ stdenv.mkDerivation {
     '';
     license = licenses.zlib;
     maintainers = [ maintainers.jpdoyle ];
-
-    platforms = platforms.linux ++ platforms.darwin;
+    platforms = platforms.linux;
   };
 }