[bug] Fix missing legacy iconfont assets referenced by netjsongraph.css

Describe the bug

src/css/netjsongraph.css defines an @font-face rule that references external legacy iconfont assets:

• iconfont.eot
• iconfont.woff
• iconfont.ttf
• iconfont.svg

None of these files are tracked in the repository or included in the generated dist/ output. The rule also includes an inline base64 WOFF2 font, so modern browsers render the icon correctly without requesting the missing files.

However, this breaks Django deployments using manifest static storage, including OpenWISP's CompressStaticFilesStorage. During collectstatic, Django resolves every CSS url() reference and fails because the external font files do not exist:

ValueError: The file 'netjsongraph/css/src/iconfont.eot' could not be found with <openwisp_utils.storage.CompressStaticFilesStorage ...>

Steps To Reproduce

1. Install or package netjsongraph.js for use in a Django deployment with manifest static storage.
2. Run collectstatic with OpenWISP's CompressStaticFilesStorage (or another storage backend that resolves CSS asset references).
3. Observe that processing netjsongraph.css fails when Django attempts to resolve the missing iconfont.eot reference.

Expected behavior

The distributed stylesheet should not reference assets that are absent from the package. The @font-face declaration should retain the embedded WOFF2 data URI while removing the unavailable EOT, WOFF, TTF, and SVG fallback URLs.

The generated dist/ output should be rebuilt after the source change.

Screenshots

Not applicable.

Start with src/css/netjsongraph.css and inspect the @font-face declaration referencing the missing iconfont assets. Retain the embedded WOFF2 data URI while removing the unavailable EOT, WOFF, TTF, and SVG URLs, then rebuild the generated dist/ output. Confirm the distributed stylesheet no longer references absent files and can be processed by manifest static storage.