remove unused code for leveldown in backend package

Author: bitmold

packages/backend/leveldown.js

@@ -201,7 +201,6 @@
   },
   "overrides": {
     "level": "$level",
-    "leveldown": "^6",
     "@orbitdb/core": {
       "multiformats": "13.3.1",
       "@ipld/dag-cbor": "9.2.1"