refactor!: stop using import_shims

Attempting to import packages from
lms/djangoapps, cms/djangoapps, or common/djangoapps
as if they are import roots will now
simply raise ImportErrors (like any other invalid
import) instead of DeprecatedEdxPlatformImportError.

See docs/decisions/0007-sys-path-modification-removal.rst
for more details.

scripts/xsslint_config.py

@@ -30,7 +30,6 @@ SKIP_DIRS = (
     'spec',
     'test_root',
     'vendor',
-    'import_shims',
 )