Sema: Remove unused -fix-string-to-substring-conversion flag

lib/Frontend/CompilerInvocation.cpp

@@ -201,12 +201,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
                           const FrontendOptions &FrontendOpts) {
   using namespace options;
 
-  /// FIXME: Remove this flag when void subscripts are implemented.
-  /// This is used to guard preemptive testing for the fix-it.
-  if (Args.hasArg(OPT_fix_string_substring_conversion)) {
-    Opts.FixStringToSubstringConversions = true;
-  }
-
   if (auto A = Args.getLastArg(OPT_swift_version)) {
     auto vers = version::Version::parseVersionString(
       A->getValue(), SourceLoc(), &Diags);