Remove redundant assignments to arc_c

Several assignments to arc_c had no effect because it is ultimately
initialized to arc_c_max.

This aligns ZoL better with the upstream code which removed these
assignments some time ago.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Signed-off-by: Tim Chase <tim@onlight.com>
Closes #5081