staging: rtl8192u: Remove defined but unused function

This patch fixes "symbol 'dm_change_rxpath_selection_setting'
was not declared. Should it be static?" sparse warning in
r8192U_dm.c After setting it as static, it causes to unused
function sparse warning, because it is not used anywhere.

Signed-off-by: Ebru Akagunduz <ebru.akagunduz@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>