[SPARK-54769][PYTHON] Remove dead code in conversion.py

### What changes were proposed in this pull request?

Dead code is removed and replaced with assertions.

### Why are the changes needed?

There are two kinds of dead code in this PR:
1. The else case. We already confirmed that we need a converter for this `dataType`, `else` case should not ever happen, unless we made a mistake in our code.
2. If `ArrayType` has an `elementType` that does not need a converter, the `ArrayType` itself should not need a converter and we should never ask for one.

assertion has clear semantics that this should never happen, but also provides a reasonable and reproducible fallback behavior when unexpected happens. We can catch our code failure faster.

### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

CI

### Was this patch authored or co-authored using generative AI tooling?

No

Closes #53536 from gaogaotiantian/remove-dead-code.

Authored-by: Tian Gao <gaogaotiantian@hotmail.com>
Signed-off-by: Hyukjin Kwon <gurwls223@apache.org>

Author: gaogaotiantian

python/pyspark/sql/conversion.py

@@ -436,11 +436,8 @@ def convert_other(value: Any) -> Any:
                 return value
 
             return convert_other
-        else:
-            if none_on_identity:
-                return None
-            else:
-                return lambda value: value
+        else:  # pragma: no cover
+            assert False, f"Need converter for {dataType} but failed to find one."
 
     @staticmethod
     def convert(data: Sequence[Any], schema: StructType, use_large_var_types: bool) -> "pa.Table":
@@ -613,23 +610,16 @@ def convert_struct(value: Any) -> Any:
                 dataType.elementType, none_on_identity=True, binary_as_bytes=binary_as_bytes
             )
 
-            if element_conv is None:
-
-                def convert_array(value: Any) -> Any:
-                    if value is None:
-                        return None
-                    else:
-                        assert isinstance(value, list)
-                        return value
-
-            else:
+            assert (
+                element_conv is not None
+            ), f"_need_converter() returned True for ArrayType of {dataType.elementType}"
 
-                def convert_array(value: Any) -> Any:
-                    if value is None:
-                        return None
-                    else:
-                        assert isinstance(value, list)
-                        return [element_conv(v) for v in value]
+            def convert_array(value: Any) -> Any:
+                if value is None:
+                    return None
+                else:
+                    assert isinstance(value, list)
+                    return [element_conv(v) for v in value]
 
             return convert_array
 
@@ -793,11 +783,8 @@ def convert_geometry(value: Any) -> Any:
 
             return convert_geometry
 
-        else:
-            if none_on_identity:
-                return None
-            else:
-                return lambda value: value
+        else:  # pragma: no cover
+            assert False, f"Need converter for {dataType} but failed to find one."
 
     @overload
     @staticmethod