Revert "libaudit: limit to 5 selinux denials per sec"

The shamu instabilities continued even after throttling SELinux denials
to 5/second. 5 denials per second is too low when doing device bringup,
and there have been some complaints about lost SELinux denials. See,
for example, http://comments.gmane.org/gmane.comp.security.selinux/21941

Bring the limit back up to 20/second to prevent dropping too many
denials on the floor.

This reverts commit a15db51b.

(cherrypick of commit 9667a660)

Change-Id: I05e85cce0a792d05aa557fcc614c0fc019c15014