xfs: remove redundant assignment to variable bit

Variable bit is being assigned a value that is never read, hence
the assignment is redundant and can be removed. Cleans up clang
warning:

fs/xfs/libxfs/xfs_rtbitmap.c:675:3: warning: Value stored to
'bit' is never read

Signed-off-by: Colin Ian King <colin.king@canonical.com>
Reviewed-by: Darrick J. Wong <darrick.wong@oracle.com>
Signed-off-by: Darrick J. Wong <darrick.wong@oracle.com>