kbuild: remove leftover comment for filechk utility

After commit 43fee2b2 ("kbuild: do not redirect the first
prerequisite for filechk"), the rule is no longer automatically passed
$< as stdin, so remove the stale comment.

Fixes: 43fee2b2 ("kbuild: do not redirect the first prerequisite for filechk")
Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Masahiro Yamada <masahiroy@kernel.org>