[docs] Remove Gitter and Slack (fixes #3689) (#3710)

README.md

@@ -12,8 +12,6 @@ Light Gradient Boosting Machine
 [![Python Versions](https://img.shields.io/pypi/pyversions/lightgbm.svg?logo=python&logoColor=white)](https://pypi.org/project/lightgbm)
 [![PyPI Version](https://img.shields.io/pypi/v/lightgbm.svg?logo=pypi&logoColor=white)](https://pypi.org/project/lightgbm)
 [![CRAN Version](https://www.r-pkg.org/badges/version/lightgbm)](https://cran.r-project.org/package=lightgbm)
-[![Join Gitter at https://gitter.im/Microsoft/LightGBM](https://badges.gitter.im/Microsoft/LightGBM.svg)](https://gitter.im/Microsoft/LightGBM?utm_source=badge&utm_medium=badge&utm_campaign=pr-badge&utm_content=badge)
-[![Slack](https://lightgbm-slack-autojoin.herokuapp.com/badge.svg)](https://lightgbm-slack-autojoin.herokuapp.com)
 
 LightGBM is a gradient boosting framework that uses tree based learning algorithms. It is designed to be distributed and efficient with the following advantages:
 
@@ -104,9 +102,6 @@ Support
 -------
 
 - Ask a question [on Stack Overflow with the `lightgbm` tag](https://stackoverflow.com/questions/ask?tags=lightgbm), we monitor this for new questions.
-- Discuss on the [LightGBM Gitter](https://gitter.im/Microsoft/LightGBM).
-- Discuss on the [LightGBM Slack team](https://lightgbm.slack.com).
-  - Use [this invite link](https://lightgbm-slack-autojoin.herokuapp.com/) to join the team.
 - Open **bug reports** and **feature requests** (not questions) on [GitHub issues](https://github.com/microsoft/LightGBM/issues).
 
 How to Contribute