Remove error from AndroidMkData.Custom

It's never anything except nil, and it unnecessarily complicates
the implementations.

Test: m -j checkbuild
Change-Id: I021c7971ede3e11bbe08cf1601f4690ed4d1a036
diff --git a/phony/phony.go b/phony/phony.go
index 79f3a1b..053ed56 100644
--- a/phony/phony.go
+++ b/phony/phony.go
@@ -49,14 +49,12 @@
 }
 
 func (p *phony) AndroidMk() (ret android.AndroidMkData, err error) {
-	ret.Custom = func(w io.Writer, name, prefix, moduleDir string) error {
+	ret.Custom = func(w io.Writer, name, prefix, moduleDir string) {
 		fmt.Fprintln(w, "\ninclude $(CLEAR_VARS)")
 		fmt.Fprintln(w, "LOCAL_PATH :=", moduleDir)
 		fmt.Fprintln(w, "LOCAL_MODULE :=", name)
 		fmt.Fprintln(w, "LOCAL_REQUIRED_MODULES := "+strings.Join(p.requiredModuleNames, " "))
 		fmt.Fprintln(w, "include $(BUILD_PHONY_PACKAGE)")
-
-		return nil
 	}
 	return
 }