modpost: delete stale comment

Commit 7840fea2 ("kbuild: Fix computing srcversion for modules")
fixed the comment above parse_source_files to refer to the new source_
line, but left this one behind that could still give the impression that
drivers/net/dummy.c appears in the deps_ variable.

Signed-off-by: Rasmus Villemoes <linux@rasmusvillemoes.dk>
Signed-off-by: Masahiro Yamada <yamada.masahiro@socionext.com>