staging: rtl8723bs: core: rtw_wlan_util: Remove unused variable 'start_seq'

staging: rtl8723bs: core: rtw_wlan_util: Remove unused variable 'start_seq'

Fixes the following W=1 kernel build warning(s):

 drivers/staging/rtl8723bs/core/rtw_wlan_util.c: In function ‘process_addba_req’:
 drivers/staging/rtl8723bs/core/rtw_wlan_util.c:1774:11: warning: variable ‘start_seq’ set but not used [-Wunused-but-set-variable]
 from drivers/staging/rtl8188eu/core/rtw_wlan_util.c:12:

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: linux-staging@lists.linux.dev
Signed-off-by: Lee Jones <lee.jones@linaro.org>
Link: https://lore.kernel.org/r/20210414181129.1628598-20-lee.jones@linaro.org
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>