It's not strictly needed there, and just complicates the
code. LZ encoder never even had this feature.
The primary reason to have uncompressed size tracking in
filter encoders was validating that the application
doesn't give different amount of input that it had
promised. A side effect was to validate internal workings
of liblzma.
Uncompressed size tracking is still present in the Block
encoder. Maybe it should be added to LZMA_Alone and raw
encoders too. It's simpler to have one coder just to
validate the uncompressed size instead of having it
in every filter.
