perf(NcAvatar): ensure avatar does not add mutation observer

[ShGKme]

☑️ Resolves

• Performance regression from: perf(utils): ensure only 64px or 512px avatars are loaded #6749
• It adds a mutation observer for every single NcAvatar to support theme change
• It is not a part of the PR "ensure only 64px or 512px avatars are loaded"
• It can be handled by NcThemeProvider for context theme change

🏁 Checklist

• ⛑️ Tests are included or are not applicable
• 📘 Component documentation has been extended, updated or is not applicable
• 2️⃣ Backport to stable8 for maintained Vue 2 version or not applicable

[ShGKme]

/backport to stable8