bsd-user/bsdload.c: Remove write-only id_change variable

bsd-user/bsdload.c: Remove write-only id_change variable

On OpenBSD the compiler complains:
bsd-user/bsdload.c:54:17: warning: variable 'id_change' set but not used [-Wunused-but-set-variable]

This is dead code that was originally copied from linux-user.
We fixed this in linux-user in commit 331c23b5ca44293d1 in 2011;
delete the useless code from bsd-user too.

Signed-off-by: Peter Maydell <peter.maydell@linaro.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>
Message-id: 1500395194-21455-4-git-send-email-peter.maydell@linaro.org