Remove prefill calibration

[abhinaykukkadapu]

To help with the long calibration issue, @haowhsu-quic helped by removing prefill calibration and copying the decode quant params to prefill which should help with significant time taken during prefill.

cc @cccclai @winskuo-quic @shewu-quic @haowhsu-quic @DannyYuyang-quic @cbilgin