remove `ConnectionContext.is_tls` from pyi
remove `ConnectionContext.is_tls` from pyi

Summary: The implementation was removed in D6899968 (https://github.com/facebook/fbthrift/commit/534493777a573661714ab5d0f8cd8680be62232f) but pyi wasn't cleaned up.

Reviewed By: yfeldblum

Differential Revision: D23443810

fbshipit-source-id: 8867bdcf874d59b60961b764512d9c41dbd1a0bb

facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift,facebook/fbthrift