fs/binfmt_elf.c: delete stale comment

"passed_fileno" variable was deleted 11 years ago in 2.6.25.

Link: http://lkml.kernel.org/r/20190529201747.GA23248@avx2
Fixes: d20894a2

 ("Remove a.out interpreter support in ELF loader")
Signed-off-by: Alexey Dobriyan <adobriyan@gmail.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>