Revert "Handle zap_add() failures in mixed ... "

This reverts commit cc63068e95ee725cce03b1b7ce50179825a6cda5.

Under certain circumstances this change can result in an ENOSPC
error when adding new files to a directory. See #7401 for full
details.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tony Hutter <hutter2@llnl.gov>
Issue #7401
Cloes #7416