Remove mention that AuthorName can be set in the app's source code

In the Build Metadata Reference, there is a warning mentioning that AuthorName entries can be set in an app’s source code.

However, looking at the documentation about metadata, it seems that there is currently no way to specify AuthorName entries in an app’s source code. The fdroid-server code for parsing fastlane and Triple-T metadata indeed only looks for AuthorEmail, AuthorPhone, and AuthorWebsite entries.

This MR removes this warning to make it clear that AuthorName cannot be set in an app’s source code.