gradle: remove 'java' plugin
gradle: remove 'java' plugin

Only one plugin of 'java' of 'java-library' is required.

Flowdalic/Smack,vanitasvitae/Smack,vanitasvitae/Smack,vanitasvitae/Smack,Flowdalic/Smack,Flowdalic/Smack,igniterealtime/Smack,igniterealtime/Smack,igniterealtime/Smack