config: remove unused pid option

This removes the code around '--pid, -p' which is not used at all. The
command-line option is still available to not break CRIU users, but as
the result has not been used, just  remove the code assigning it.

Signed-off-by: Adrian Reber <areber@redhat.com>
Signed-off-by: Andrei Vagin <avagin@virtuozzo.com>