Remove unused imports default_theme SCsub

[Anutrix]

Removes unused imports from default_theme's SCsub script.

Vulture(https://pypi.org/project/vulture/):

find . -name "*.py" -o -name "SCsub" -o -name "SConstruct" | xargs vulture | grep "unused import"

showed 0 results.

LGTM(https://lgtm.com/projects/g/godotengine/godot/alerts?mode=list&severity=&id=py%2Funused-import) had just those 2 unused imports in default_theme's SCsub script.

Autoflake(https://pypi.org/project/autoflake/) that uses pyflakes:

find . -name "*.py" -o -name "SCsub" -o -name "SConstruct" | xargs vulture | grep "unused import"

showed above and 4 more results in 3 more files.

Do suggest any other tool for static analysis.

[Calinou]

It might be worth checking for unused imports in all Python files (**/*.py, **/SCsub, **/SConstruct).

[Anutrix]

It might be worth checking for unused imports in all Python files (**/*.py, **/SCsub, **/SConstruct).
Done.
Updated main comment.

[akien-mga]

Thanks!

[akien-mga]

Cherry-picked for 3.4.