Suggested code cleanup for CWG-C059

What version / commit were you testing with?

d3ac1111

Issue description?

I noticed some oddities with the code of CWG-C059. And based on my offline discussion with @hegilmez , we should cleanup the code as follows:

(1) The macro: #define MODE_DEPTX_TXSIZES 19 in enums.h should more generally be #define MODE_DEPTX_TXSIZES TX_SIZES_ALL. Even better, remove MODE_DEPTX_TXSIZES macro and use TX_SIZES_ALL directly.

(2) According to the Proposed Method section in the input document:

DCT_DCT is used as the only transform for block sizes where max(width, height) is either equal to 32 or 64.

So, the last two entries in av1_size_class array should be 3 (instead of 2), as they correspond to TX_16X64 and TX_64X16 sizes.

@hegilmez confirmed that this correction will not affect the bitstream, but it's good to make the change so that code is more consistent.