selftests: ublk: remove useless 'delay_us' from 'struct dev_ctx'

selftests: ublk: remove useless 'delay_us' from 'struct dev_ctx'

'delay_us' shouldn't be added to 'struct dev_ctx' since now it is
handled by per-target command line & 'struct fault_inject_ctx'.

So remove it.

Fixes: 81586652bb1f ("selftests: ublk: add generic_06 for covering fault inject")
Signed-off-by: Ming Lei <ming.lei@redhat.com>
Reviewed-by: Uday Shankar <ushankar@purestorage.com>
Link: https://lore.kernel.org/r/20250421235947.715272-3-ming.lei@redhat.com
Signed-off-by: Jens Axboe <axboe@kernel.dk>