LATX, fix: Remove TU_UNLINK_STUB_INVALID
Since jmp_target_arg and tu_unlink are in the same union, initializing
tu_unlink with TU_UNLINK_STUB_INVALID may cause some errors. Currently,
the validity of tu_unlink can be determined using use_tu_jmp(), therefore
UNLINK_STUB_INVALID is being removed.