Drop unneeded system header includes

<memory.h> is a non-standard obsolete header that was long ago
replaced by <string.h>.

<malloc.h> is a non-standard header; it is not obsolete (we must
use it for malloc_trim, for example), but generally should not
be used in files that just need malloc() and friends, where
<stdlib.h> is the standard header.

And since osdep.h already guarantees string.h and stdlib.h, we
can drop these unusual system header includes as redundant
rather than replacing them.

Signed-off-by: Eric Blake <eblake@redhat.com>
Signed-off-by: Michael Tokarev <mjt@tls.msk.ru>
Reviewed-by: Philippe Mathieu-Daudé <f4bug@amsat.org>
Reviewed-by: Thomas Huth <thuth@redhat.com>