Remove declarations for unused search_label() local variables

Remove declarations for unused search_label() local variables
error, i, p and poff. Fewer complaints from gcc -Wall.

The same was done to the NetBSD version in 2011 by tsutui.

Remove two more recent load_disklabel() comments implying that
search_label() checks MBR partitions. It doesn't and never has
despite the enticing names/types of the above unused variables.

No functional change.