[CIR] Fix warning in CIRGenAction.cpp (#118389)

Fix a compiler warning in `CIRGenConsumer::HandleTranslationUnit` in
`clang/lib/CIR/FrontendAction/CIRGenAction.cpp`. The warning was about a
`default:` section in a switch statement that already covered all the
values of an enum. Delete the `default:` section.

Author: dkolsen-pgi

clang/lib/CIR/FrontendAction/CIRGenAction.cpp

@@ -66,9 +66,6 @@ class CIRGenConsumer : public clang::ASTConsumer {
         MlirModule->print(*OutputStream, Flags);
       }
       break;
-    default:
-      llvm_unreachable("NYI: CIRGenAction other than EmitCIR");
-      break;
     }
   }
 };