Remove redundant "publishNonDefault" setting

I think I've seen the following message from just about every single
gradle run throughout the upgrade process:

publishNonDefault is deprecated and has no effect anymore. All variants are now published.

app/build.gradle

@@ -31,7 +31,6 @@ ext {
 android {
     compileSdkVersion 27
     buildToolsVersion '28.0.3'
-    publishNonDefault true
 
     defaultConfig {
         applicationId "eu.kanade.tachiyomi"