devel/binutils: Remove autodependency on devel/elfutils

Users who manually pre-install devel/elfutils and subsequently
manually build and install binutils must now enable the dependency.

The previous commit provided the option but also didn't change the
implied (autodetect) behavior. This change now disables elfutils
autodetection. Users must now explicitly enable the option if they
were previously relying on autodetection.

Requested by: mat, jrm