staging: r8188eu: hal data's board type is unused

BoardType in struct hal_data_8188e is not used. Remove the variable and the
function to extract the board type from the efuses.

Signed-off-by: Martin Kaiser <martin@kaiser.cx>
Link: https://lore.kernel.org/r/20211125151436.5895-7-martin@kaiser.cx


Signed-off-by: Greg Kroah-Hartman <gregkh@linuxfoundation.org>