### What changes were proposed in this pull request?
This removes the undocumented and incomplete feature of "stateful aggregation" in continuous mode, which would reduce 1100+ lines of code.
### Why are the changes needed?
The work for the feature had been stopped for over an year, and no one asked/requested for the availability of such feature in community. Current state for the feature is that it only works with `coalesce(1)` which force the query to read and process, and write in "a" task, which doesn't make sense in production.
The remaining code increases the work on DSv2 changes as well - that's why I don't simply propose reverting relevant commits - the code path has been changed due to DSv2 evolution.
### Does this PR introduce _any_ user-facing change?
Technically no, because it's never documented and can't be used in production in current shape.
### How was this patch tested?
Existing tests.
Closes#29077 from HeartSaVioR/SPARK-31985.
Authored-by: Jungtaek Lim (HeartSaVioR) <kabhwan.opensource@gmail.com>
Signed-off-by: Jungtaek Lim (HeartSaVioR) <kabhwan.opensource@gmail.com>
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