Fix zap_update() ASSERT from ztest

This patch simply removes an invalid assert from the zap_update()
function. The ASSERT is invalid because it does not hold the zap
lock from the time it fetches the old value to the time it confirms
that it is what it should be.

Reviewed by: Matt Ahrens <matt@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tom Caputi <tcaputi@datto.com>
Closes #8209