tests: Remove bcvk VM dispatch from Rust integration tests

tests: Remove bcvk VM dispatch from Rust integration tests

VM deployment is now handled externally by `just test-tmt` (bcvk + tmt)
or `just integration-container` (bcvk direct SSH). The Rust test binary
runs inside the VM as root, so the require_root/RunMode dispatch logic
is no longer needed.

- Remove require_root(), RunMode enum, and bcvk dispatch code
- Simplify booted_test! and privileged_test! macros to just register
  and run tests directly
- Remove rustix dependency (no longer checking getuid)

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
Signed-off-by: Xiaofeng Wang <henrywangxf@me.com>