bitops: remove duplicated extern declarations

If CONFIG_GENERIC_FIND_NEXT_BIT is enabled, find_next_bit() and
find_next_zero_bit() are doubly declared in asm-generic/bitops/find.h
and linux/bitops.h.

asm/bitops.h includes asm-generic/bitops/find.h if and only if the
architecture enables CONFIG_GENERIC_FIND_NEXT_BIT. And asm/bitops.h
is included by linux/bitops.h

So we can just remove the extern declarations of find_next_bit() and
find_next_zero_bit() in linux/bitops.h.

Also we can remove unneeded #ifndef CONFIG_GENERIC_FIND_NEXT_BIT in
asm-generic/bitops/find.h.

Signed-off-by: Akinobu Mita <akinobu.mita@gmail.com>
Signed-off-by: Arnd Bergmann <arnd@arndb.de>