Remove deprecated zfs_arc_p_aggressive_disable

zfs_arc_p_aggressive_disable is no more. This PR removes docs
and module parameters for zfs_arc_p_aggressive_disable.

Reviewed-by: Giuseppe Di Natale <dinatale2@llnl.gov>
Reviewed by: George Melikov <mail@gmelikov.ru>
Reviewed-by: Brian Behlendorf <behlendorf1@llnl.gov>
Richard Elling <Richard.Elling@RichardElling.com>
Closes #7135