Remove bogus check in check_features

The check_features RPC function required that both known fields are
present. Without those fields it exited with an error. If RPC users
where not specifying all parameters it would fail. It should, however,
be possible to only check for a subset of options.

Each supported option is checked separately anyway in the forked criu
which does the actual check.

Removing the check also enables RPC clients with older protobuf
definitions to use the feature check.

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