From Android Studio Preferences,(My Android Studio version 3.6.1)
Select Version Control->Select git Path-> Click ‘ – ‘ (minus button)->OK
Windows: File > Settings > Version Control > Select Project > ‘-‘ (Minus Button) >Apply > Ok
UPDATE:
Currently google updated this settings.So if you want to delete GitHub connection from your project Open android Studio Settings ->Version Control ->Github -> click ‘ – ‘ (minus button) -> Apply -> OK
