Revert "Use OCP classes as much as possible in files_external"

apps/files_external/lib/amazons3.php

apps/files_external/lib/api.php

apps/files_external/lib/config.php

apps/files_external/lib/dropbox.php

apps/files_external/lib/google.php

apps/files_external/lib/swift.php

apps/files_external/service/storagesservice.php

apps/files_external/templates/settings.php

apps/files_external/tests/mountconfig.php