arc: Drop an incorrect assert

Unfortunately, there was an overzealous assertion that was (in pretty
specific circumstances) false, causing failure. This assertion was
added in error, so we're removing it.

Reviewed-by: Matthew Ahrens <mahrens@delphix.com>
Reviewed-by: George Wilson <gwilson@delphix.com>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Rich Ercolani <rincebrain@gmail.com>
Closes #9897
Closes #12020
Closes #12246