Remove unused ArrayRef::is_arrow (#8868)

## Summary

- remove the unused `ArrayRef::is_arrow` helper
- keep this reviewer-requested cleanup separate from #8864

## Testing

- `cargo check -p vortex-array`
- `cargo clippy --all-targets --all-features` (pre-push hook)

Signed-off-by: Connor Tsui <connor.tsui20@gmail.com>

Author: connortsui20

vortex-array/src/array/erased.rs

@@ -34,15 +34,10 @@ use crate::array::ArrayId;
 use crate::array::ArrayInner;
 use crate::array::ArraySlots;
 use crate::array::DynArrayData;
-use crate::arrays::Bool;
 use crate::arrays::Constant;
 use crate::arrays::DictArray;
 use crate::arrays::FilterArray;
-use crate::arrays::Null;
-use crate::arrays::Primitive;
 use crate::arrays::SliceArray;
-use crate::arrays::VarBin;
-use crate::arrays::VarBinView;
 use crate::buffer::BufferHandle;
 use crate::builders::ArrayBuilder;
 use crate::dtype::DType;
@@ -443,15 +438,6 @@ impl ArrayRef {
         nbytes
     }
 
-    /// Returns whether this array is an arrow encoding.
-    pub fn is_arrow(&self) -> bool {
-        self.is::<Null>()
-            || self.is::<Bool>()
-            || self.is::<Primitive>()
-            || self.is::<VarBin>()
-            || self.is::<VarBinView>()
-    }
-
     /// Whether the array is of a canonical encoding.
     pub fn is_canonical(&self) -> bool {
         self.is::<AnyCanonical>()