Remove fbprophet package from PyPi and conda forge

Hello!

I've seen some confusion around which package (fbprophet/prophet) is actually the right one to install.

I think fbprophet should be removed from PyPi and conda forge if it is no longer being maintained.

The requirements.txt/conda recipe for fbprophet lists `pystan>=2.14` as a dependency, which will install an incompatible pystan 3 package and error out during package install. This causes a lot of confusion to users, especially when they install fbprophet along with other packages.

You can repro this like follow:

- Download the fbprophet tar ball [here](https://pypi.org/project/fbprophet/#files)
- Open requirements.txt

You should see the following:

```
Cython>=0.22
cmdstanpy==0.9.5
pystan>=2.14
numpy>=1.15.4
pandas>=1.0.4
matplotlib>=2.0.0
LunarCalendar>=0.0.9
convertdate>=2.1.2
holidays>=0.10.2
setuptools-git>=1.2
python-dateutil>=2.8.0
tqdm>=4.36.1
```

pystan>=2.14 will install the latest pystan (pystan 3). However, Prophet does not support pystan 3!!

You can see the same thing in the conda [recipe](https://github.com/conda-forge/fbprophet-feedstock/blob/master/recipe/meta.yaml#L34).