[woff2] Remove compression type support

The spec supports only Brötli compression, and the code always set it
already.
[woff2] Use the names from the spec in the message
[woff2] Make table decompression less confusing

There is only a single data stream for all the compressed tables, so why
write code as if each table is compressed separately and add conditions
everywhere to circumvent this?
[woff2] Remove the bogus compressed_sum stuff

We do the correct check few lines below.