Remove `SingleStepGdbBehavior::Unknown` variant

[daniel5151]

With https://github.com/daniel5151/gdbstub/pull/95, there are no longer any Arch implementation in gdbstub_arch that use SingleStepGdbBehavior::Unknown, and therefore, this variant can be removed.

Doing so would be a breaking change, and would only land as part of the next breaking release of gdbstub (which at the time of writing would be gdbstub 0.7).

This is a tracking issue to make sure this variant gets removed at some point prior to 0.7's release.