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/cc/androidmk.go b/cc/androidmk.go
index f68ddbb..561a0a3 100644
--- a/cc/androidmk.go
+++ b/cc/androidmk.go
@@ -131,7 +131,7 @@
 
 		ret.Class = "SHARED_LIBRARIES"
 	} else if library.header() {
-		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)
@@ -158,8 +158,6 @@
 
 			library.androidMkWriteExportedFlags(w)
 			fmt.Fprintln(w, "include $(BUILD_HEADER_LIBRARY)")
-
-			return nil
 		}
 
 		return
@@ -191,13 +189,11 @@
 }
 
 func (object *objectLinker) AndroidMk(ctx AndroidMkContext, ret *android.AndroidMkData) {
-	ret.Custom = func(w io.Writer, name, prefix, moduleDir string) error {
+	ret.Custom = func(w io.Writer, name, prefix, moduleDir string) {
 		out := ret.OutputFile.Path()
 
 		fmt.Fprintln(w, "\n$("+prefix+"OUT_INTERMEDIATE_LIBRARIES)/"+name+objectExtension+":", out.String())
 		fmt.Fprintln(w, "\t$(copy-file-to-target)")
-
-		return nil
 	}
 }