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/python/androidmk.go b/python/androidmk.go
index 386eeee..ab24e99 100644
--- a/python/androidmk.go
+++ b/python/androidmk.go
@@ -38,10 +38,12 @@
 	}
 }
 
-func (p *pythonBaseModule) AndroidMk() (ret android.AndroidMkData, err error) {
+func (p *pythonBaseModule) AndroidMk() android.AndroidMkData {
+	ret := android.AndroidMkData{}
+
 	p.subAndroidMk(&ret, p.installer)
 
-	return ret, nil
+	return ret
 }
 
 func (p *pythonBinaryHostDecorator) AndroidMk(base *pythonBaseModule, ret *android.AndroidMkData) {