Remove unused legacy HCI command serializers

btsnd_hcic_accept_cis_req
btsnd_hcic_accept_conn
btsnd_hcic_ble_enh_rx_test
btsnd_hcic_ble_enh_tx_test
btsnd_hcic_change_conn_type
btsnd_hcic_rej_cis_req

Bug: 331817295
Test: m com.android.btservices
Flag: EXEMPT, dead code removal
Change-Id: I79d5fec628d40780e660e5bf3612b29f024cae8f