bsd-user: remove a.out support

bsd-user: remove a.out support

Remove still-born a.out support. The BSDs switched from a.out to ELF 20+ years
ago. It's out of scope for bsd-user, and what little support there was would
simply wind up at a not-implemented message. Simplify the whole mess by removing
it entirely. Should future support be required, it would be better to start from
scratch.

Signed-off-by: Warner Losh <imp@bsdimp.com>
Reviewed-by: Richard Henderson <richard.henderson@linaro.org>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>