ath10k: switch testmode to use ath10k_core_fetch_firmware_api_n()

Now that all firmware-N.bin related are within struct ath10k_fw_file we can
switch to use ath10k_core_fetch_firmware_api_n() and delete almost identical
ath10k_tm_fetch_utf_firmware_api_2().

Signed-off-by: Kalle Valo <kvalo@qca.qualcomm.com>