Remove unused error prone
Remove unused error prone

Nothing happened since one year in the error prone repo concerning Java 9+

sauliusg/jabref,JabRef/jabref,Siedlerchr/jabref,JabRef/jabref,sauliusg/jabref,sauliusg/jabref,Siedlerchr/jabref,sauliusg/jabref,JabRef/jabref,Siedlerchr/jabref,Siedlerchr/jabref,JabRef/jabref