taging: rtl8723au: Remove empty function odm_Init_RSSIForDM23a

The function odm_Init_RSSIForDM23a is empty and therefore there is no use
of keeping it. Also, remove its prototype.

Signed-off-by: Bhumika Goyal <bhumirks@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>