MB-23905: getWithMeta doesn't need to bgFetch for datatype

MB-23905: getWithMeta doesn't need to bgFetch for datatype

Some more code left-over from when datatype was part of the value
is now removed. getMeta only needs to perform a meta-data fetch
and have no special case for datatype requests.

A test is added which recreates what happened to trigger the MB.
A getMeta was returning key_enoent instead of the datatype because
it was doing a full bgFetch against a deleted value.

Change-Id: I6715d789f6cb8503cd44b860fd78ae3224d9bc67
Reviewed-on: http://review.couchbase.org/77182
Reviewed-by: Dave Rigby <daver@couchbase.com>
Tested-by: Build Bot <build@couchbase.com>