remove block-device tests from Commands.pm and Sizes.pm

These tests are already performed in Volumes.pm and even if they weren't this
is the wrong spot to test the property. (The correct location is either while
analyzing the current hardware configuration or just before actually applying
the generated commands - because that's when the devices +must+ be present.)

During parsing and solving it does not matter at all if the device is a block
device or even present. It just makes it harder or even impossible to verify
the correct parsing / solving of configs intended for other hardware
configurations.