Remove unused get_gpu_num import (fixes flake8 F401)

The import was left over after refactoring the geotiff test to use @params
instead of a manual GPU check.

Author: shreyaskommuri

dali/test/python/decoder/test_image.py

@@ -27,7 +27,6 @@
 from nose_utils import assert_raises, SkipTest
 from test_utils import compare_pipelines
 from test_utils import get_dali_extra_path
-from test_utils import get_gpu_num
 from test_utils import get_nvjpeg_ver
 from test_utils import to_array