chore: Fix merge conflicts from merging comet-parquet-exec into main

[andygrove]

Which issue does this PR close?

N/A

Rationale for this change

Fix issues from #1318

What changes are included in this PR?

• Remove some orphaned source files
• Revert a documentation change

How are these changes tested?

[parthchandra]

thanks @andygrove

[comphead]

lgtm thanks @andygrove

[kazuyukitanimura]

@andygrove @parthchandra @comphead
I still see more merge issues

Looks like common/src/main/scala/org/apache/spark/sql/comet/util/Utils.scala and common/src/main/scala/org/apache/spark/sql/comet/CometArrowUtils.scala are unmerged/duplicates

[kazuyukitanimura]

native/core/src/parquet/parquet_support.rs and native/spark-expr/src/conversion_funcs/cast.rs are also unmerged/duplicates

I think we really should work on revert the this commit and previous commit and do a proper merge
@andygrove @parthchandra @comphead

[parthchandra]

@kazuyukitanimura I don't see duplicates of any of the above files. Can you clarify what you mean by unmerged?

[kazuyukitanimura]

@parthchandra E.g. for the utils file fromArrowType is defined in both files

Another potential issue
spark/src/test/scala/org/apache/spark/sql/comet/CometPlanStabilitySuite.scala](https://github.com/apache/datafusion-comet/blob/main/spark/src/test/scala/org/apache/spark/sql/comet/CometPlanStabilitySuite.scala#L349
is testing only when the mode is SCAN_NATIVE_COMET?

[kazuyukitanimura]

https://github.com/apache/datafusion-comet/blob/main/spark/src/main/scala/org/apache/comet/CometSparkSessionExtensions.scala#L1267

looks like offheap mode check is re-introduced

[parthchandra]

@parthchandra E.g. for the utils file fromArrowType is defined in both files

Oh, I get it now. We did duplicate code in the feature branch (CometArrowUtils and Utils both duplicate code from Spark. Let me clean this up in a followup)
The cast.rs and parquet_support.rs duplication is intentional. We used the former as the starting point for parquet type conversions. The parquet_support code will change as we address more data types.

Another potential issue spark/src/test/scala/org/apache/spark/sql/comet/CometPlanStabilitySuite.scala](https://github.com/apache/datafusion-comet/blob/main/spark/src/test/scala/org/apache/spark/sql/comet/CometPlanStabilitySuite.scala#L349 is testing only when the mode is SCAN_NATIVE_COMET?

Yes. The plans are different when the scans are different. We generated new plans for the different scan types but did not turn them on. The two new scan implementations do not yet pass all the unit tests so turning on plan stability for them is a little premature

[kazuyukitanimura]

https://github.com/apache/datafusion-comet/blob/main/native/spark-expr/src/variance.rs and https://github.com/apache/datafusion-comet/blob/main/native/spark-expr/src/agg_funcs/variance.rs

[andygrove]

https://github.com/apache/datafusion-comet/blob/main/native/spark-expr/src/variance.rs and https://github.com/apache/datafusion-comet/blob/main/native/spark-expr/src/agg_funcs/variance.rs

This is already being addressed in #1323