Remove unused import

This wasn't reported by pylint due to a pylint bug (apparently):
`pylint A B` doesn't complain about an unused import in B if A happens to
import and use the same module, which happens to be the case when we run
pylint on the CI.

Signed-off-by: Gilles Peskine <Gilles.Peskine@arm.com>

tests/scripts/generate_bignum_tests.py

@@ -60,7 +60,6 @@ from abc import ABCMeta
 from typing import List
 
 import scripts_path # pylint: disable=unused-import
-from mbedtls_dev import test_case
 from mbedtls_dev import test_data_generation
 from mbedtls_dev import bignum_common
 # Import modules containing additional test classes