Remove apache-beam from dependencies to prevent Colab install error

[JiaweiZhuang]

When installing this package in Colab via !pip install git+https://github.com/google-research/data-driven-pdes.git, I got

ERROR: Command errored out with exit status 1: python setup.py egg_info Check the logs for full command output.

due to the apache-beam dependency.

See this notebook for the complete install error:
https://colab.research.google.com/drive/1UbKevNe8BIPExuBdiqW3ZWy58Lh6E1gk

Since the beam pipeline is rarely used in notebook anyways, I just removed the beam dependency to fix the problem.

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