[operator tools] Add forbiddennapis plugin to pom.xml so that we can permanently ban references to hbase I.A. private classes from hbck2

Following suggestion from elserj on PR for HBASE-23791, this adds forbiddenapis plugin to hbck2 pom.xml file, defining a ban to MetaTableAccessor. This also gets rid of one remaining reference to MetaTableAccessor on HBCKMetaTableAccessor that we missed to detect in HBASE-23791.