[LTO] Remove Config.UseDefaultPipeline (#82587)

This option is not used. It was added in
[D122133](https://reviews.llvm.org/D122133), 5856f30, with the only
usage in `ClangLinkerWrapper.cpp`, which was later updated in a1d57fc,
and then finally removed in [D142650](https://reviews.llvm.org/D142650),
6185246.

Author: igorkudrin

llvm/include/llvm/LTO/Config.h

@@ -60,9 +60,6 @@ struct Config {
   bool VerifyEach = false;
   bool DisableVerify = false;
 
-  /// Use the standard optimization pipeline.
-  bool UseDefaultPipeline = false;
-
   /// Flag to indicate that the optimizer should not assume builtins are present
   /// on the target.
   bool Freestanding = false;

llvm/lib/LTO/LTOBackend.cpp

@@ -330,8 +330,6 @@ static void runNewPMPasses(const Config &Conf, Module &Mod, TargetMachine *TM,
       report_fatal_error(Twine("unable to parse pass pipeline description '") +
                          Conf.OptPipeline + "': " + toString(std::move(Err)));
     }
-  } else if (Conf.UseDefaultPipeline) {
-    MPM.addPass(PB.buildPerModuleDefaultPipeline(OL));
   } else if (IsThinLTO) {
     MPM.addPass(PB.buildThinLTODefaultPipeline(OL, ImportSummary));
   } else {