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/genrule/genrule.go b/genrule/genrule.go
index 18ec0a4..95df169 100644
--- a/genrule/genrule.go
+++ b/genrule/genrule.go
@@ -63,7 +63,7 @@
 	ctx.RegisterModuleType("genrule", GenRuleFactory)
 
 	ctx.FinalDepsMutators(func(ctx android.RegisterMutatorsContext) {
-		ctx.BottomUp("genrule_tool_deps", toolDepsMutator).Parallel()
+		ctx.BottomUp("genrule_tool_deps", toolDepsMutator)
 	})
 }