get rid of `ESConnector.prototype.removeMappings`

[pulkitsinghal]

If removing an index also effectively removes any mappings and data then there shouldn't be any need for an explicit method such as ESConnector.prototype.removeMappings ... we need to confirm this by reading ES docs for various versions and also running sanity tests.

If this turns out to be true then we can cleanup the code by removing ESConnector.prototype.removeMappings