[NFC][CodeGen] Remove dead ParallelCG.h/.cpp API (llvm#95770)

LTOBackend inlined it a while ago and now uses a static copy. This API
was unused.

We can always restore it at some point if it's needed, but right now
it's just bloat.

Author: Pierre-vh

llvm/include/llvm/CodeGen/ParallelCG.h

@@ -163,7 +163,6 @@ add_llvm_component_library(LLVMCodeGen
   MacroFusion.cpp
   NonRelocatableStringpool.cpp
   OptimizePHIs.cpp
-  ParallelCG.cpp
   PeepholeOptimizer.cpp
   PHIElimination.cpp
   PHIEliminationUtils.cpp

llvm/lib/CodeGen/CMakeLists.txt

@@ -20,7 +20,6 @@
 #include "llvm/Analysis/TargetTransformInfo.h"
 #include "llvm/Bitcode/BitcodeWriter.h"
 #include "llvm/CodeGen/CommandFlags.h"
-#include "llvm/CodeGen/ParallelCG.h"
 #include "llvm/CodeGen/TargetSubtargetInfo.h"
 #include "llvm/Config/config.h"
 #include "llvm/IR/Constants.h"

llvm/lib/CodeGen/ParallelCG.cpp

@@ -170,7 +170,6 @@ static_library("CodeGen") {
     "OptimizePHIs.cpp",
     "PHIElimination.cpp",
     "PHIEliminationUtils.cpp",
-    "ParallelCG.cpp",
     "PatchableFunction.cpp",
     "PeepholeOptimizer.cpp",
     "PostRAHazardRecognizer.cpp",