Remove MutatorHandle.Parallel()

All mutators are parallel by default, remove the explicit Parallel()
calls.

Bug: 372540665
Test: all soong tests pass
Flag: EXEMPT refactor
Change-Id: I41e3a71bd13c75b7fceae91b1c4cfe678ab62df2
diff --git a/android/namespace.go b/android/namespace.go
index 866d125..8b3ebc4 100644
--- a/android/namespace.go
+++ b/android/namespace.go
@@ -457,7 +457,7 @@
 }
 
 func RegisterNamespaceMutator(ctx RegisterMutatorsContext) {
-	ctx.BottomUp("namespace_deps", namespaceMutator).Parallel().MutatesGlobalState()
+	ctx.BottomUp("namespace_deps", namespaceMutator).MutatesGlobalState()
 }
 
 func namespaceMutator(ctx BottomUpMutatorContext) {