Remove deprecated BOARD_BUILD_SYSTEM_ROOT_IMAGE

This variable is always false or empty now, so clean up remaining
references of the variable.

Remove "buildvariant=" kernel commandline as it was used by
system-as-root kernels to determine the system build variant. It is
unsupported now.

Bug: 241346584
Test: Presubmit
Change-Id: I0f97b6e0bf8b03429f65366119ff78f9c7983516