staging: rtl8192u: remove unused functions in r8192U_core.c

We get 2 warnings when building kernel with W=1:
drivers/staging/rtl8192u/r8192U_core.c:925:12: warning: no previous declaration for 'ieeerate2rtlrate' [-Wmissing-declarations]
drivers/staging/rtl8192u/r8192U_core.c:958:12: warning: no previous declaration for 'rtl8192_rate2rate' [-Wmissing-declarations]
drivers/staging/rtl8192u/r8192U_core.c:1322:11: warning: no previous declaration for 'rtl8192_IsWirelessBMode' [-Wmissing-declarations]

In fact, these functions are unused in
r8192U_core.c, but should be removed.

So this patch removes the unused functions.

Signed-off-by: Baoyou Xie <baoyou.xie@linaro.org>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>