staging: rtl8188eu: remove unused field bRxRSSIDisplay in struct adapter

Since commit da25a8ec ("staging: rtl8188eu: remove unused
debugging functions") this field is never set set to any nonzero
value, so it is actually always zero.

It is also used only once, as a boolean inside an if(). This means the
code under that if() is dead code, so remove it as well.

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>