Remove Garnett as a Testing Dependency

[tommy-waltmann]

Description

This PR removes garnett from freud's test suite.

Motivation and Context

We plan to support gsd>=3.0.0 in the future and garnett cannot read gsd>=3.0 files due to changes in gsd's read/write mode flags.

How Has This Been Tested?

Garnett has been removed from usage in freud's tests, so if the remaining 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.

[tommy-waltmann]

This should be merged after #1132

[b-butler]

Looks fine to me. Do we still plan to advertise garnett support given its unmaintained status?

[tommy-waltmann]

Looks fine to me. Do we still plan to advertise garnett support given its unmaintained status?

Let's make an issue for it. I don't have the bandwidth to address this right now, but at some point docs pages should be updated.

[tommy-waltmann]

issue created in #1142