Remove unused llvm/Support/Parallel.h from MLIR (NFC)

This header aren't needed anymore: MLIR is using a thread pool
injected in the context instead of a global one.

mlir/lib/IR/Verifier.cpp

@@ -33,7 +33,6 @@
 #include "mlir/IR/Threading.h"
 #include "llvm/ADT/StringMap.h"
 #include "llvm/Support/FormatVariadic.h"
-#include "llvm/Support/Parallel.h"
 #include "llvm/Support/PrettyStackTrace.h"
 #include "llvm/Support/Regex.h"
 #include <atomic>

mlir/lib/Pass/Pass.cpp

@@ -23,7 +23,6 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/CrashRecoveryContext.h"
 #include "llvm/Support/Mutex.h"
-#include "llvm/Support/Parallel.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/Threading.h"
 #include "llvm/Support/ToolOutputFile.h"

mlir/lib/Pass/PassCrashRecovery.cpp

@@ -18,7 +18,6 @@
 #include "llvm/Support/CommandLine.h"
 #include "llvm/Support/CrashRecoveryContext.h"
 #include "llvm/Support/Mutex.h"
-#include "llvm/Support/Parallel.h"
 #include "llvm/Support/Signals.h"
 #include "llvm/Support/Threading.h"
 #include "llvm/Support/ToolOutputFile.h"

mlir/lib/Transforms/Inliner.cpp

@@ -22,7 +22,6 @@
 #include "mlir/Transforms/Passes.h"
 #include "llvm/ADT/SCCIterator.h"
 #include "llvm/Support/Debug.h"
-#include "llvm/Support/Parallel.h"
 
 #define DEBUG_TYPE "inlining"