cam_periph: Remove vestigial "scbus" comparison

The code in camperiphunit rejects "scbus" as an 'at' location that would
allow any other wiring to use that unit number. Yet in
camperiphunitnext, if we have a no target and the 'at' location of
'scbus' it would be excluded on the basis that it's a wiring
cadidate. This is improper and appears to be a hold-over of the
pre-hints / pre-newbus config system, so remove it.

Sponsored by: Netflix
Differential Revision: https://reviews.freebsd.org/D32680