fix(storage): Try to delete existing target

[solracsf]

Summary

Should get rid of logs like:

{
  "reqId": "iq8oEiCJ1Uc7WrCf9uNP",
  "level": 3,
  "app": "PHP",
  "method": "GET",
  "url": "/index.php/apps/files/api/v1/thumbnail/256/256/00_COMMUN/ORGAN/EST-LES/File.pdf",
  "message": "unlink(/var/www/data/appdata_ocwfs52oscxg/preview/d/a/d/3/f/6/3/4158280/256-256-crop.webp): No such file or directory at /var/www/nextcloud/lib/private/Files/Storage/Local.php#348",
  "userAgent": "Mozilla/5.0 (Android) Nextcloud-android/3.29.2",
  "version": "29.0.9.2",
  "data": {
    "app": "PHP"
  },
}

Checklist

• Code is properly formatted
• Sign-off message is added to all commits
• Tests (unit, integration, api and/or acceptance) are included
• Screenshots before/after for front-end changes
• Documentation (manuals or wiki) has been updated or is not required
• Backports requested where applicable (ex: critical bugfixes)