a.out: remove define-only CMAGIC, previously magic number

The last user was removed in 5.1 in
commit 08300f44 ("a.out: remove core dumping support")
but this is part of the UAPI headers, so this may want to either wait
until a.out is removed entirely, or be removed from the magic number doc
and silently remain in the header

A cursory glance on DCS didn't show any user code actually using this
value

Found with
grep MAGIC Documentation/process/magic-number.rst | while read -r mag _;
do git grep -wF "$mag"  | grep -ve '^Documentation.*magic-number.rst:' \
-qe ':#define '"$mag" || git grep -wF "$mag" | while IFS=: read -r f _;
do sed -i '/\b'"$mag"'\b/d' "$f"; done ; done

Signed-off-by: Ahelenia Ziemiańska <nabijaczleweli@nabijaczleweli.xyz>
Link: https://lore.kernel.org/r/9cbea062df7125ef43e2e0b2a67ede6ad1c5f27e.1663280877.git.nabijaczleweli@nabijaczleweli.xyz


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>