refactor(quickadapter): drop unused sample_weighting tunables

refactor(quickadapter): drop unused sample_weighting tunables

The sample_weighting.{aggregation,softmax_temperature} options were inert:
LABEL_COLUMNS is single-label by design and compose_sample_weights only
ever sees one per-label vector, making row-wise aggregation an identity
operation regardless of the configured mode.

Removes the config block, validation specs, getter, and README entry;
compose_sample_weights keeps its kwargs with safe defaults (arithmetic_mean,
T=1.0) so the call site stays trivial.