l2arc_write_buffers: remove redundant asserts

Probably introduced inadvertently in b525630 (Native Encryption).

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: George Amanakis <gamanakis@gmail.com>
Signed-off-by: Christian Schwarz <christian.schwarz@nutanix.com>
Closes #12935