hterm: have removeAllKnownHosts return true
hterm: have removeAllKnownHosts return true

Users who don't write javascript and use the console aren't accustomed to
seeing "undefined" as a synonym for "success".

BUG=None
TEST=ran term_.command.removeAllKnownHosts() in my console

Change-Id: I0a46918155cc284543cdec40362b21b172126712
Reviewed-on: https://gerrit.chromium.org/gerrit/41018
Reviewed-by: Robert Ginda <4419ac4e35d191308e944202d542b192188656d1@chromium.org>
Commit-Queue: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>
Tested-by: Mike Frysinger <8f3f75c74bd5184edcfa6534cab3c13a00a2f794@chromium.org>

hernad/libapps,hernad/libapps,hernad/libapps,hernad/libapps