Fix mujoco frameskip = 0 leftover

[belerico]

Summary

This PR solves #94. In particular:

• frame_skip=0 is removed from the make_dict_env method
• frame_skip has also been removed from the DMCWrapper. Action repeat can still being applied by setting env.action_repeat>=1

Type of Change

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• Bug fix (non-breaking change that solves an issue)

Checklist

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• I have tested my changes locally and they work as expected. (Please describe the tests you performed.)
• I have added unit tests for my changes, or updated existing tests if necessary.
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