Fix Filen icon (#426)

Fix Filen SVG

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## Description

- Fix Filen SVG that was showing like that on my iPhone:

![image](https://github.com/ente-io/auth/assets/98350888/e543cf82-2d46-48df-b846-57fba0b42945)

- Remove GitLab slug that was not necessary (GitLab = gitlab)

<!--- Describe your changes in detail -->

## Type of Change

<!--- Put an `x` in all the boxes that apply: -->

- [ ] 🖼️ New icon
- [ ] ✨ New feature (non-breaking change which adds functionality)
- [x] 🛠️ Bug fix (non-breaking change which fixes an issue)
- [ ] ❌ Breaking change (fix or feature that would cause existing
functionality to change)
- [ ] 🧹 Code refactor
- [ ] ✅ Build configuration change
- [ ] 📝 Documentation
- [ ] 🗑️ Chore

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Co-authored-by: Vishnu Mohandas <vishnu@ente.io>

assets/custom-icons/_data/custom-icons.json

@@ -85,8 +85,7 @@
       "hex": "858585"
     },
     {
-      "title": "GitLab",
-      "slug": "gitlab"
+      "title": "GitLab"
     },
     {
       "title": "Google"