sysutils/nut-devel: Disable automatic enphase API discovery

Disable automatic enphase API discovery. Any port that enables
the enphase API will result in a pkg-plist error. We do not need
the external enphase API.

Though the documentation states that external enphase API discovery is
disabled, the nut build enables it automatically. Building it in
a clean jail without unneeded ports installed will not discover this
error.