Drop old page fault related stats.
Drop old page fault related stats.

We don't show them on the UI anymore. And luckily I didn't find them
documented.

Change-Id: Ie08df3951e3abc888a4aa86383eca38dcfe5de73
Reviewed-on: http://review.couchbase.org/46913
Tested-by: Aliaksey Artamonau <3c875bcfb3adf2a65b2ae7686ca921e6c9433147@gmail.com>
Reviewed-by: Aliaksey Kandratsenka <340b8e09ca65cd3fc686427fcfed17e87eaf61e2@gmail.com>

ceejatec/ns_server,mschoch/ns_server,ceejatec/ns_server,membase/ns_server,mschoch/ns_server,nimishzynga/ns_server,mschoch/ns_server,mschoch/ns_server,membase/ns_server,mschoch/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,ceejatec/ns_server,mschoch/ns_server,nimishzynga/ns_server,ceejatec/ns_server,membase/ns_server,membase/ns_server,ceejatec/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,nimishzynga/ns_server,membase/ns_server