xfs: Remove unused function is_rt_data_fork

xfs: Remove unused function is_rt_data_fork

The function are defined in the rmap_repair.c file, but not called
elsewhere, so delete the unused function.

fs/xfs/scrub/rmap_repair.c:436:1: warning: unused function 'is_rt_data_fork'.

Reported-by: Abaci Robot <abaci@linux.alibaba.com>
Closes: https://bugzilla.openanolis.cn/show_bug.cgi?id=8425
Signed-off-by: Jiapeng Chong <jiapeng.chong@linux.alibaba.com>
Reviewed-by: "Darrick J. Wong" <djwong@kernel.org>
Signed-off-by: Chandan Babu R <chandanbabu@kernel.org>