libtommath: remove obsolete `TARGET_OS_*` workaround

pkgs/by-name/li/libtommath/package.nix

···
 
   makefile = "makefile.shared";
 
-  env.NIX_CFLAGS_COMPILE = lib.optionalString (
-    stdenv.hostPlatform.isDarwin && stdenv.hostPlatform.isAarch64
-  ) "-DTARGET_OS_IPHONE=0";
-
   enableParallelBuilding = true;
 
   meta = with lib; {