GH-46439: [C++] Remove unneeded namespace prefix in test_util_internal.h (#46695)

### Rationale for this change

Addresses #46180 (comment). One of three PRs to resolve #46439.

### What changes are included in this PR?

- Remove unneeded namespace prefix in test_util_internal.h.

### Are these changes tested?

Yes. Impacted tests still pass.

### Are there any user-facing changes?

No.
* GitHub Issue: #46439

Authored-by: Bryce Mecum <[email protected]>
Signed-off-by: Sutou Kouhei <[email protected]>

Author: amoeba

cpp/src/arrow/dataset/test_util_internal.h

@@ -2139,8 +2139,8 @@ class WriteFileSystemDatasetMixin : public MakeFileSystemDatasetMixin {
         actual_struct = std::dynamic_pointer_cast<Array>(struct_array);
       }
 
-      auto expected_struct = arrow::ArrayFromJSON(
-          struct_(expected_physical_schema_->fields()), file_contents->second);
+      auto expected_struct = ArrayFromJSON(struct_(expected_physical_schema_->fields()),
+                                           file_contents->second);
 
       AssertArraysEqual(*expected_struct, *actual_struct, /*verbose=*/true);
     }