os-posix: Drop misleading comment

Comments explaining why we include a header tend to go bad.  This
one's almost comical: not only doesn't qemu-options.hx use
MAP_POPULATE anymore (since commit ef36fa1, v2.0.0, 2013), even the
include it applies to got moved away in commit 02d0e09 (v2.7.0).

Signed-off-by: Markus Armbruster <armbru@redhat.com>
Reviewed-by: Marc-André Lureau <marcandre.lureau@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>

os-posix.c

@@ -25,7 +25,6 @@
 
 #include "qemu/osdep.h"
 #include <sys/wait.h>
-/*needed for MAP_POPULATE before including qemu-options.h */
 #include <pwd.h>
 #include <grp.h>
 #include <libgen.h>