kdump: Remove unneeded (and removed) validation

Previously the validation was called to ensure that a value read from a
config file was valid. Due to changes the plugins.validate() method is
not available anymore and the check was additionally not necessary.

Change-Id: Ied731e09db97f4692a35ba1f32c98715d886461a
Signed-off-by: Fabian Deutsch <fabiand@fedoraproject.org>