staging: rtlwifi: Remove unused functions

Remove the five functions that are not used from the source and header
files. Issue found by Coccinelle's semantic patch results for
returnvar.cocci.

Signed-off-by: Maya Nakamura <m.maya.nakamura@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>