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