Remove dead link to old slack channel

Removed user support link to the old slack channel. We can probably replace it with something else, but I think it is better to stop users from hoping too much

Author: Gabrielgerez

README.md

@@ -14,7 +14,6 @@ chemistry (Hartree-Fock and Density Functional Theory).
 The code is being developed at the Hylleraas Centre for Quantum Molecular
 Sciences at UiT - The Arctic University of Norway.
 
-### User support: [mrchem.slack.com](https://join.slack.com/t/mrchem/shared_invite/enQtNTI3MjMzNjM0NTk0LWNkODZjNTMwYmM4NmRmODExMjQzMDc3NThlMzNmNmIyNWQwM2YwOGY0OWY4NmNmNzE4ZmM2NzgxYzUzNDg3NDM)
 ### Documentation: [mrchem.readthedocs.io](http://mrchem.readthedocs.io)