Remove error from AndroidMkData.Extra

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

Test: m -j checkbuild
Change-Id: I8a117a86aa39aeb07d9d8d0686ef869c52784f19
diff --git a/python/androidmk.go b/python/androidmk.go
index 0a79e73..386eeee 100644
--- a/python/androidmk.go
+++ b/python/androidmk.go
@@ -61,7 +61,7 @@
 		ret.OutputFile = android.OptionalPathForPath(installer.path)
 	}
 
-	ret.Extra = append(ret.Extra, func(w io.Writer, outputFile android.Path) error {
+	ret.Extra = append(ret.Extra, func(w io.Writer, outputFile android.Path) {
 		path := installer.path.RelPathString()
 		dir, file := filepath.Split(path)
 		stem := strings.TrimSuffix(file, filepath.Ext(file))
@@ -69,6 +69,5 @@
 		fmt.Fprintln(w, "LOCAL_MODULE_SUFFIX := "+filepath.Ext(file))
 		fmt.Fprintln(w, "LOCAL_MODULE_PATH := $(OUT_DIR)/"+filepath.Clean(dir))
 		fmt.Fprintln(w, "LOCAL_MODULE_STEM := "+stem)
-		return nil
 	})
 }