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

fs/xfs/libxfs/xfs_rtbitmap.c

@@ -672,7 +672,6 @@ xfs_rtmodify_range(
 		/*
 		 * Compute a mask of relevant bits.
 		 */
-		bit = 0;
 		mask = ((xfs_rtword_t)1 << lastbit) - 1;
 		/*
 		 * Set/clear the active bits.