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/variable.go b/android/variable.go
index 417ba89..4f6e3ff 100644
--- a/android/variable.go
+++ b/android/variable.go
@@ -29,7 +29,7 @@
 
 func registerVariableBuildComponents(ctx RegistrationContext) {
 	ctx.PreDepsMutators(func(ctx RegisterMutatorsContext) {
-		ctx.BottomUp("variable", VariableMutator).Parallel()
+		ctx.BottomUp("variable", VariableMutator)
 	})
 }