The gather path attended over the selected latents with a plain
matmul and softmax. It only ran with n_ubatch <= 16, and the flash
attention backends now skip the masked rows through n_kv_max, so
the scatter path covers every case.

Drop the gather flag, the gathered attention branch and
gather_mla_rows. set_input_kpool always maps padding to the n_kv
sentinel, and the slot mask becomes sel_mask since only the
scatter reads it.