Makefile: remove bogus comment

The comment "Check files that are touched by more than one package"
was previously located right before the calls to the check-uniq-files
script. However, this script and the logic calling it have been
removed in commit 2496189a ("core:
drop check-uniq-files"), so the comment no longer makes any sense:
let's drop it.

Signed-off-by: Thomas Petazzoni <thomas.petazzoni@bootlin.com>
Signed-off-by: Yann E. MORIN <yann.morin.1998@free.fr>