Remove last traces of a.out support from ELF loader.

In commit d20894a2 ("Remove a.out
interpreter support in ELF loader"), Andi removed support for a.out
interpreters from the ELF loader, which was only ever needed for the
transition from a.out to ELF.

This removes the last traces of that support, in particular the
inclusion of <linux/a.out.h>.

Signed-off-by: David Woodhouse <dwmw2@infradead.org>
Acked-by: Peter Korsgaard <jacmet@sunsite.dk>
Signed-off-by: Linus Torvalds <torvalds@linux-foundation.org>