CDRIVER-629 remove MONGOC_WRITE_CONCERN_W_ERRORS_IGNORED

[kevinAlbs]

Summary

Remove the MONGOC_WRITE_CONCERN_W_ERRORS_IGNORED (-1) value, which is equivalent to MONGOC_WRITE_CONCERN_W_UNACKNOWLEDGED (0).

Notes

I was surprised to discover mongoc_write_concern_is_valid returns true for unsupported values of w. This PR does not change this behavior. The server returns an error for a negative w value:

$ mongosh --eval "db.runCommand({insert: 'c', documents: [{}], writeConcern: {w: -1}})"
MongoServerError: w has to be a non-negative number and not greater than 50; found: -1

Adding a client-side error might go against the Where possible, depend on server to return errors mantra.