Update README.md (#1962)

Removing slack link

README.md

@@ -7,7 +7,6 @@
 [![pre-commit](https://img.shields.io/badge/pre--commit-enabled-brightgreen?logo=pre-commit&logoColor=white)](https://github.com/pre-commit/pre-commit)
 [![Python 3.7, 3.8](https://img.shields.io/badge/python-3.6%20%7C%203.7%20%7C%203.8-blue.svg)](https://www.python.org/downloads/release/)
 [![Supports Bullet](https://img.shields.io/static/v1?label=supports&message=Bullet%20Physics&color=informational&link=https://opensource.google/projects/bullet3)](https://opensource.google/projects/bullet3)
-[![Slack Join](http://img.shields.io/static/v1?label=Join%20us%20on&message=%23habitat-dev&labelColor=%234A154B&logo=slack)](https://join.slack.com/t/ai-habitat/shared_invite/enQtNjY1MzM1NDE4MTk2LTZhMzdmYWMwODZlNjg5MjZiZjExOTBjOTg5MmRiZTVhOWQyNzk0OTMyN2E1ZTEzZTNjMWM0MjBkN2VhMjQxMDI)
 [![Twitter Follow](https://img.shields.io/twitter/follow/ai_habitat?style=social)](https://twitter.com/ai_habitat)
 
 # Habitat-Sim