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/cc/sanitize.go b/cc/sanitize.go
index 1fcb32c..4ea4d26 100644
--- a/cc/sanitize.go
+++ b/cc/sanitize.go
@@ -444,12 +444,10 @@
 }
 
 func (sanitize *sanitize) AndroidMk(ctx AndroidMkContext, ret *android.AndroidMkData) {
-	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) {
 		if sanitize.androidMkRuntimeLibrary != "" {
 			fmt.Fprintln(w, "LOCAL_SHARED_LIBRARIES += "+sanitize.androidMkRuntimeLibrary)
 		}
-
-		return nil
 	})
 }