[Caching] Remove temporary caching test option

Remove `-allow-unstable-cache-key-for-testing` frontend flag. It is a
test only flag when the infrastructure is not ready to write tests for
fully cachable tasks. It is no longer needed after all the related tests
are rewritten to use dependency scanner.

Author: cachemeifyoucan

include/swift/Option/FrontendOptions.td

@@ -1273,9 +1273,6 @@ def always_compile_output_files :
   HelpText<"Always compile output files even it might not change the results">;
 
 // CAS/Caching related options.
-def allow_unstable_cache_key_for_testing: Flag<["-"], "allow-unstable-cache-key-for-testing">,
-  HelpText<"Allow compilation caching with unstable inputs for testing purpose">;
-
 def input_file_key : Separate<["-"], "input-file-key">,
   HelpText<"Cache Key for input file">;
 def bridging_header_pch_key : Separate<["-"], "bridging-header-pch-key">,

lib/Frontend/ArgsToFrontendOptionsConverter.cpp

@@ -275,10 +275,8 @@ bool ArgsToFrontendOptionsConverter::convert(
   if (Opts.EnableCaching && Opts.CASFSRootIDs.empty() &&
       Opts.ClangIncludeTrees.empty() &&
       FrontendOptions::supportCompilationCaching(Opts.RequestedAction)) {
-    if (!Args.hasArg(OPT_allow_unstable_cache_key_for_testing)) {
-        Diags.diagnose(SourceLoc(), diag::error_caching_no_cas_fs);
-        return true;
-    }
+    Diags.diagnose(SourceLoc(), diag::error_caching_no_cas_fs);
+    return true;
   }
 
   if (FrontendOptions::doesActionGenerateIR(Opts.RequestedAction)) {