[Pipelines] Drop outdated comments about O0 (NFC)

These methods can be called with an O0 level nowadays.

Author: nikic

llvm/include/llvm/Passes/PassBuilder.h

@@ -231,11 +231,6 @@ class PassBuilder {
   /// optimization and code generation without any link-time optimization. It
   /// typically correspond to frontend "-O[123]" options for optimization
   /// levels \c O1, \c O2 and \c O3 resp.
-  ///
-  /// Note that \p Level cannot be `O0` here. The pipelines produced are
-  /// only intended for use when attempting to optimize code. If frontends
-  /// require some transformations for semantic reasons, they should explicitly
-  /// build them.
   ModulePassManager buildPerModuleDefaultPipeline(OptimizationLevel Level,
                                                   bool LTOPreLink = false);
 
@@ -246,11 +241,6 @@ class PassBuilder {
   /// a ThinLTO run. It works to minimize the IR which needs to be analyzed
   /// without making irreversible decisions which could be made better during
   /// the LTO run.
-  ///
-  /// Note that \p Level cannot be `O0` here. The pipelines produced are
-  /// only intended for use when attempting to optimize code. If frontends
-  /// require some transformations for semantic reasons, they should explicitly
-  /// build them.
   ModulePassManager buildThinLTOPreLinkDefaultPipeline(OptimizationLevel Level);
 
   /// Build an ThinLTO default optimization pipeline to a pass manager.