Remove dead code in build/codegen.go.

Remove the `art.codegenCustomizer` type definition, which is not used
anywhere.

Test: m
Change-Id: I645237f8980f99d2e3f81c42e344ecb4f7808c0e
diff --git a/build/codegen.go b/build/codegen.go
index a9966f2..cc967ac 100644
--- a/build/codegen.go
+++ b/build/codegen.go
@@ -164,11 +164,6 @@
 	}
 }
 
-type codegenCustomizer struct {
-	library           bool
-	codegenProperties codegenProperties
-}
-
 func defaultDeviceCodegenArches(ctx android.LoadHookContext) []string {
 	arches := make(map[string]bool)
 	for _, a := range ctx.DeviceConfig().Arches() {