(DOCS-15131): Available rc is no longer default (#705)

Author: jeff-allen-mongo

source/reference/read-concern-available.txt

@@ -18,10 +18,6 @@ A query with read concern ``"available"`` returns data from the instance
 with no guarantee that the data has been written to a majority of the
 replica set members (i.e. may be rolled back).
 
-Read concern ``"available"`` is the default for reads against secondaries
-if the reads are not associated with :ref:`causally consistent sessions
-<sessions>`.
-
 **For a sharded cluster**, :readconcern:`"available"` read concern
 provides greater tolerance for partitions since it does not wait to
 ensure consistency guarantees. That is, read concern