headers: Drop SCM_MAX_FD internal variable

This limit is not exported by a kernel, so no need
to "explicitly" define it pretending that it's immutable.

Still I kept this name in comment just to give reader
a tip where CR_SCM_MAX_FD came from.

Signed-off-by: Cyrill Gorcunov <gorcunov@openvz.org>
Signed-off-by: Pavel Emelyanov <xemul@parallels.com>