staging: rtl8188eu: remove unused function _linked_rx_signal_strehgth_display

Not referenced anymore since commit 9fe7b29c ("staging: rtl8188eu:
remove unused field bRxRSSIDisplay in struct adapter").

Cc: Greg Kroah-Hartman <gregkh@linuxfoundation.org>
Cc: Larry Finger <Larry.Finger@lwfinger.net>
Cc: Ivan Safonov <insafonov@gmail.com>
Cc: devel@driverdev.osuosl.org
Signed-off-by: Luca Ceresoli <luca@lucaceresoli.net>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>