Remove duplicate code in l2arc_evict()

l2arc_evict() performs the adjustment of the size of buffers to be
written on L2ARC unnecessarily. l2arc_write_size() is called right
before l2arc_evict() and performs those adjustments.

Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Reviewed-by: Brian Atkinson <batkinson@lanl.gov>
Signed-off-by: George Amanakis <gamanakis@gmail.com>
Closes #14828