Inline `filename` in `url`

[jakirkham]

The source/fn entry is unneeded and can be dropped. Once dropped, the url can just inline filename to simplify the recipe. This will make things a bit easier on the version update bot and align with boost-cpp, which is making the same change ( https://github.com/conda-forge/boost-cpp-feedstock/pull/133 ).

xref: https://github.com/regro/cf-scripts/issues/1588

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Checklist

• [ ] Used a personal fork of the feedstock to propose changes
• [ ] Bumped the build number (if the version is unchanged)
• [ ] Reset the build number to 0 (if the version changed)
• [ ] Re-rendered with the latest conda-smithy (Use the phrase code>@<space/conda-forge-admin, please rerender in a comment in this PR for automated rerendering)
• [ ] Ensured the license file is being packaged.

[conda-forge-webservices[bot]]

Hi! This is the friendly automated conda-forge-linting service.

I just wanted to let you know that I linted all conda-recipes in your PR (recipe) and found it was in an excellent condition.