ath79: teltonika-rut230: drop reg_usb_vbus

ath79: teltonika-rut230: drop reg_usb_vbus

USB VBUS regulator was attached to GPIO19, which isn't in fact
controlling the modem power itself, but rather modem power key - which
has to be asserted high for at least 500ms, to start the modem. Keeping
it high allows the modem to reboot upon shutdown - so it is desirable to
control this line from userspace, for example - to allow clean modem
shutdown down upon powering off the router part.

Signed-off-by: Lech Perczak <lech.perczak@gmail.com>
Link: https://github.com/openwrt/openwrt/pull/17503
Signed-off-by: Hauke Mehrtens <hauke@hauke-m.de>