staging: wilc1000: remove host_int_get_rx_power_level

This patch removes host_int_get_rx_power_level function definition and
declaration that is defined at host_interface.c and host_interface.h.
This function is defined but not used anywhere in this driver so just
remove it.

Signed-off-by: Chaehyun Lim <chaehyun.lim@gmail.com>
Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>