x86/efi: Remove unused find_bits() function

Left behind by commit fc372064 ("efi/libstub: Move Graphics Output
Protocol handling to generic code").

Signed-off-by: Lukas Wunner <lukas@wunner.de>
Cc: Ard Biesheuvel <ard.biesheuvel@linaro.org>
Signed-off-by: Matt Fleming <matt@codeblueprint.co.uk>