Remove error from AndroidMkDataProvider.AndroidMk

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

Test: m -j checkbuild
Change-Id: I3e3b7251f32ffa84dbdfd0448faf248c306ca808
diff --git a/phony/phony.go b/phony/phony.go
index 053ed56..5f0187f 100644
--- a/phony/phony.go
+++ b/phony/phony.go
@@ -48,13 +48,14 @@
 	}
 }
 
-func (p *phony) AndroidMk() (ret android.AndroidMkData, err 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)")
+func (p *phony) AndroidMk() android.AndroidMkData {
+	return android.AndroidMkData{
+		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
 }