Remove codecov

[tommy-waltmann]

Description

This PR removes codecov from freud's requirements files.

Motivation and Context

The codecov package no longer exists on PyPI: https://about.codecov.io/blog/message-regarding-the-pypi-package/

freud doesn't use codecov to do anything anymore, so the requirement itself is vestigial.

I first noticed this on #1096 when CI failed for unknown reasons.

How Has This Been Tested?

If CI tests pass, we are good to go.

Types of changes

• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds or improves functionality)
• Breaking change (fix or feature that would cause existing functionality to change)
• Documentation improvement (updates to user guides, docstrings, or developer docs)

Checklist:

• I have read the CONTRIBUTING document.
• My code follows the code style of this project.
• I have updated the documentation (if relevant).
• I have added tests that cover my changes (if relevant).
• All new and existing tests passed.
• I have updated the credits.
• I have updated the Changelog.