Remove unused import (#17005)

This PR removes an unused unused import in cudf which was causing errors in doc builds.

Authors:
  - Matthew Murray (https://github.com/Matt711)

Approvers:
  - Bradley Dice (https://github.com/bdice)

URL: #17005

python/cudf/cudf/_lib/string_casting.pyx

@@ -6,7 +6,6 @@ from cudf._lib.scalar import as_device_scalar
 from cudf._lib.types import SUPPORTED_NUMPY_TO_LIBCUDF_TYPES
 
 from libcpp.memory cimport unique_ptr
-from libcpp.string cimport string
 from libcpp.utility cimport move
 
 from pylibcudf.libcudf.column.column cimport column