[HBCK2] Remove removeExtraRegionsFromMeta from HBCK2 doc

HBCK2 doc mentions about an internal method name, removeExtraRegionsFromMeta, which is called from the extraRegionsInMeta --fix command internally.

https://github.com/apache/hbase-operator-tools/tree/master/hbase-hbck2#extra-regions-in-hbasemeta-regiontable-restorerebuild

Such problem can be addressed with the Master online, using the removeExtraRegionsFromMeta command in HBCK2.

We should replace with the right command.