Revert "[flang][mlir] disable mlir threading"

This reverts commit ae17d71.

Author: ronlieb

mlir/lib/IR/MLIRContext.cpp

@@ -59,8 +59,7 @@ struct MLIRContextOptions {
   llvm::cl::opt<bool> disableThreading{
       "mlir-disable-threading",
       llvm::cl::desc("Disable multi-threading within MLIR, overrides any "
-                     "further call to MLIRContext::enableMultiThreading()"),
-      llvm::cl::init(true)};
+                     "further call to MLIRContext::enableMultiThreading()")};
 
   llvm::cl::opt<bool> printOpOnDiagnostic{
       "mlir-print-op-on-diagnostic",