rtw88: drop unused rtw_coex_coex_dm_reset()

0day reports:

sparse warnings:

drivers/net/wireless/realtek/rtw88/coex.c:2457:6: sparse:
	symbol 'rtw_coex_coex_dm_reset' was not declared. Should it be static?

rtw_coex_coex_dm_reset() is not called. Remove it.

Fixes: 4136214f ("rtw88: add BT co-existence support")
Reported-by: kbuild test robot <lkp@intel.com>
Signed-off-by: Guenter Roeck <groeck@chromium.org>
Signed-off-by: Brian Norris <briannorris@chromium.org>
Signed-off-by: Kalle Valo <kvalo@codeaurora.org>