chore: remove unused imports

[parthea]

Fixes warnings from flake8 . --select F401

(py39) partheniou@partheniou-vm-2:~/git/google-api-python-client$ flake8 . --select F401
./describe.py:37:1: F401 'googleapiclient.discovery.build' imported but unused
./googleapiclient/discovery_cache/file_cache.py:30:1: F401 'threading' imported but unused
./googleapiclient/discovery_cache/__init__.py:19:1: F401 'datetime' imported but unused
./googleapiclient/discovery_cache/__init__.py:40:13: F401 'google.appengine.api.memcache' imported but unused
./scripts/buildprbody.py:15:1: F401 'enum.IntEnum' imported but unused
./scripts/buildprbody_test.py:20:1: F401 'shutil' imported but unused
./scripts/buildprbody_test.py:24:1: F401 'changesummary.ChangeType' imported but unused
./samples/adexchangeseller/get_all_preferred_deals.py:25:1: F401 'argparse' imported but unused
./samples/adexchangeseller/get_all_metrics.py:25:1: F401 'argparse' imported but unused
./samples/adexchangeseller/get_all_alerts.py:25:1: F401 'argparse' imported but unused
./samples/adexchangeseller/get_all_dimensions.py:25:1: F401 'argparse' imported but unused
./samples/storage_serviceaccount_appengine/main.py:39:1: F401 'logging' imported but unused
./samples/storage_serviceaccount_appengine/main.py:40:1: F401 'os' imported but unused
./samples/storage_serviceaccount_appengine/main.py:41:1: F401 'pickle' imported but unused
./samples/storage_serviceaccount_appengine/main.py:46:1: F401 'google.appengine.ext.webapp.template' imported but unused
./samples/analytics/management_v3_reference.py:57:1: F401 'argparse' imported but unused
./samples/analytics/hello_analytics_api_v3.py:47:1: F401 'argparse' imported but unused
./samples/appengine/main.py:29:1: F401 'logging' imported but unused
./samples/appengine/main.py:31:1: F401 'pickle' imported but unused
./apiclient/__init__.py:3:1: F401 'googleapiclient' imported but unused
./tests/test_discovery.py:43:1: F401 'google.auth.transport.mtls' imported but unused
./tests/test_discovery.py:81:1: F401 'googleapiclient.http.BatchHttpRequest' imported but unused
./tests/test_discovery_cache.py:26:1: F401 'googleapiclient.discovery_cache.base.Cache' imported but unused