fs/binfmt_elf.c: remove unneeded initialization of mm->start_stack

As pointed out by zoujc@lenovo.com, setup_arg_pages() already
initialized current->mm->start_stack.

Link: https://bugzilla.kernel.org/show_bug.cgi?id=202881
Reported-by:  <zoujc@lenovo.com>
Signed-off-by: Andrew Morton <akpm@linux-foundation.org>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>