portable: Remove unnecessary assert()

Fixes #23433

matches is plumbed through until it finally gets used in unit_match()
which can deal with NULL matches so the assert() is unnecessary and
can be removed.

The two call sites of extract_image_and_extensions() also don't
assert() on matches either.

src/portable/portable.c

@@ -530,7 +530,6 @@ static int extract_image_and_extensions(
         int r;
 
         assert(name_or_path);
-        assert(matches);
 
         r = image_find_harder(IMAGE_PORTABLE, name_or_path, NULL, &image);
         if (r < 0)