Remove unused @df.setter

[andersy005]

Change Summary

The @df.setter method isn't used/allowed by Pydantic. This PR removes it.

---------------------------------------------------------------------------
ValueError                                Traceback (most recent call last)
<ipython-input-78-2517183ee52f> in <module>
----> 1 cat.df = None

/glade/work/abanihi/opt/miniconda/envs/ecgtools-dev/lib/python3.9/site-packages/pydantic/main.cpython-39-x86_64-linux-gnu.so in pydantic.main.BaseModel.__setattr__()

ValueError: "ESMCatalogModel" object has no field "df"

Related issue number

Checklist

• Unit tests for the changes exist
• Tests pass on CI
• Documentation reflects the changes where applicable