Given a DataFrame of numeric values, subtract the row mean from each element in that row. Return the result.

Sample data: df:

      0     1     2
0   0.0   1.0   2.0
1   3.0   4.0   5.0
2   6.0   7.0   8.0
3   9.0  10.0  11.0
4  12.0  13.0  14.0