RUBY-1333 Remove obsolete message strings for write retries.
RUBY-1333 Remove obsolete message strings for write retries.

Examination of mongo 2.6 and master source code yielded that:

no master: not used as a message at all
could not contact primary: uses HostNotFound code
Not primary: use NotMaster code

All of the above codes are already covered by the code logic.

jonhyman/mongo-ruby-driver,saghm/mongo-ruby-driver,mongodb/mongo-ruby-driver,jonhyman/mongo-ruby-driver,mongodb/mongo-ruby-driver,mongodb/mongo-ruby-driver,saghm/mongo-ruby-driver