nissa: remove manual USB-A VBUS enable twiddling
nissa: remove manual USB-A VBUS enable twiddling

Commit 3fceb32f74625fca5b589410fed37740ee2d0165 enabled this with the
standard mechanism, so we can remove this temporary hack.

BUG=b:222238390
TEST=en_usb_a0_vbus GPIO automatically changes state with AP power,
     according to `gpioget en_usb_a0_vbus`
BRANCH=none

Signed-off-by: Peter Marheine <ef0b2c492edde94778d7465bfbaeed52c5be6858@chromium.org>
Change-Id: I46ab952ba2e191c490a1d10303f1e15992dd4189
Reviewed-on: https://chromium-review.googlesource.com/c/chromiumos/platform/ec/+/3544751
Reviewed-by: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@google.com>
Commit-Queue: Andrew McRae <44ad1424cfc8411c96d9181798ca41ae55709dfb@google.com>

coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec,coreboot/chrome-ec