Abolish DmTargetDefaultKey::IsLegacy

To make it easier to support disk formats created using old versions
of dm-default-key with new kernels, stop checking the kernel module
version; instead, the caller decides whether to use the old or new
options format.

Bug: 150761030
Test: crosshatch and cuttlefish boot normally; cuttlefish
    fails with "default-key: Not enough arguments" as expected when
    option is set to 1
Change-Id: I810bb7e085b9b8648858de1a02165162d1fd163e