brcm63xx: remove obsolete brcm63xx_has_reset_button

r37000 made button kmods loaded from generic code, so we don't use
it anymore for deciding to load them.

Signed-off-by: Jonas Gorski <jogo@openwrt.org>

git-svn-id: svn://svn.openwrt.org/openwrt/trunk@44838 3c298f89-4303-0410-b956-a3cf2f4a3e73