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/java/androidmk.go b/java/androidmk.go
index 92af0ed..fd967cb 100644
--- a/java/androidmk.go
+++ b/java/androidmk.go
@@ -24,9 +24,8 @@
 func (library *Library) AndroidMk() (ret android.AndroidMkData, err error) {
 	ret.Class = "JAVA_LIBRARIES"
 	ret.OutputFile = android.OptionalPathForPath(library.outputFile)
-	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) {
 		fmt.Fprintln(w, "LOCAL_MODULE_SUFFIX := .jar")
-		return nil
 	})
 	return
 }
@@ -34,9 +33,8 @@
 func (prebuilt *Import) AndroidMk() (ret android.AndroidMkData, err error) {
 	ret.Class = "JAVA_LIBRARIES"
 	ret.OutputFile = android.OptionalPathForPath(prebuilt.combinedClasspathFile)
-	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) {
 		fmt.Fprintln(w, "LOCAL_MODULE_SUFFIX := .jar")
-		return nil
 	})
 	return
 }