staging: rtl8723au: core: rtw_cmd: Remove unused functions

Removes some functions that are not used anywhere:
rtw_set_ch_cmd23a() rtw_cmd_clr_isr23a()

This was partially found by using a static code analysis program called cppcheck.

Signed-off-by: Rickard Strandqvist <rickard_strandqvist@spectrumdigital.se>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>