Remove unneccessary imports and add Checkstyle rules

src/main/java/de/thm/arsnova/config/PersistenceConfig.java

@@ -20,7 +20,6 @@ package de.thm.arsnova.config;
 
 import org.ektorp.impl.StdCouchDbInstance;
 import org.ektorp.spring.HttpClientFactoryBean;
-import org.springframework.beans.factory.annotation.Value;
 import org.springframework.boot.context.properties.EnableConfigurationProperties;
 import org.springframework.context.annotation.Bean;
 import org.springframework.context.annotation.ComponentScan;