fs/ntfs3: Remove unused function wnd_bits

Since the function wnd_bits is defined but not called in any file, it is
a useless function, and we delete it in view of the brevity of the code.

Remove some warnings found by running scripts/kernel-doc, which is
caused by using 'make W=1'.

fs/ntfs3/bitmap.c:54:19: warning: unused function 'wnd_bits' [-Wunused-function].

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Signed-off-by: Konstantin Komarov <almaz.alexandrovich@paragon-software.com>