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/rust/rust.go b/rust/rust.go
index a044a99..b22ebf7 100644
--- a/rust/rust.go
+++ b/rust/rust.go
@@ -47,11 +47,11 @@
 func registerPreDepsMutators(ctx android.RegisterMutatorsContext) {
 	ctx.Transition("rust_libraries", &libraryTransitionMutator{})
 	ctx.Transition("rust_stdlinkage", &libstdTransitionMutator{})
-	ctx.BottomUp("rust_begin", BeginMutator).Parallel()
+	ctx.BottomUp("rust_begin", BeginMutator)
 }
 
 func registerPostDepsMutators(ctx android.RegisterMutatorsContext) {
-	ctx.BottomUp("rust_sanitizers", rustSanitizerRuntimeMutator).Parallel()
+	ctx.BottomUp("rust_sanitizers", rustSanitizerRuntimeMutator)
 }
 
 type Flags struct {