Remove 2nd-order transform for first-order DC coefficients.
Since addition of the larger-scale transforms (16x16, 32x32), these don't give a benefit at macroblock-sizes anymore. At superblock-sizes, 2nd-order transform was never used over the larger transforms. Future work should test whether there is a benefit for that use case. Change-Id: I90cadfc42befaf201de3eb0c4f7330c56e33330a