Enable importing ObjC pointers as UncheckedOptional.

There's no point in maintaining a driver option for this
because of the extent of the library changes required to
make it work.

Swift SVN r15784

lib/Frontend/CompilerInvocation.cpp

@@ -538,8 +538,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
     Opts.addBuildConfigOption(A->getValue());
   }
 
-  Opts.EnableObjCOptional = Args.hasArg(OPT_enable_objc_optional);
-
   Opts.EnableAppExtensionRestrictions = Args.hasArg(OPT_enable_app_extension);
 
   if (const Arg *A = Args.getLastArg(OPT_split_objc_selectors,