fix: HashJoin panic with String dictionary keys (don't flatten keys) (apache#20505)

## Which issue does this PR close?

- Fixes apache#20696
- Follow on to apache#20441

## Rationale for this change


apache#20441 (review)
fixes the special case DictionaryArray handling in Joins. However, I
don't think we need to special case DictionaryArrays at all

## What changes are included in this PR?

1. Remove the special case dictionary handling

## Are these changes tested?

Yes by CI
## Are there any user-facing changes?

No (though maybe some queries get faster)

Author: alamb

datafusion/physical-plan/src/joins/hash_join/inlist_builder.rs

@@ -20,7 +20,6 @@
 use std::sync::Arc;
 
 use arrow::array::{ArrayRef, StructArray};
-use arrow::compute::cast;
 use arrow::datatypes::{Field, FieldRef, Fields};
 use arrow_schema::DataType;
 use datafusion_common::Result;