[Backtracing] Remove support for implicit import of _Backtracing.

We're going to rename the module to Runtime, and it isn't going to be an
implicit import, so we don't need any of this.

rdar://124913332

lib/Frontend/CompilerInvocation.cpp

@@ -939,11 +939,6 @@ static bool ParseLangArgs(LangOptions &Opts, ArgList &Args,
   Opts.DisableImplicitCxxModuleImport |=
     Args.hasArg(OPT_disable_implicit_cxx_module_import);
 
-  Opts.DisableImplicitBacktracingModuleImport =
-    Args.hasFlag(OPT_disable_implicit_backtracing_module_import,
-                 OPT_enable_implicit_backtracing_module_import,
-                 true);
-
   if (Args.hasArg(OPT_enable_experimental_async_top_level))
     Diags.diagnose(SourceLoc(), diag::warn_flag_deprecated,
                    "-enable-experimental-async-top-level");