Code cleanup suggested by Coverity or IntelliJ

Code cleanup suggested by Coverity or IntelliJ.

CommandCommandHandler - removal of unnecessary if, checked with Jean-Francois Denise
ConnectionInfoHandler - lastChainCertificate is X509Certificate[], instanceof check is not necessary
RealmIdentityManager - credential is always PasswordCredential, only null check is relevant

Second PR for removal of unused variables aka dead local store, for details see the PR https://github.com/wildfly/wildfly-core/pull/3204