Drop level-sublevel support

In preference of subleveldown

Author: vweevers

package.json

@@ -11,7 +11,6 @@
   },
   "devDependencies": {
     "bytewise": "^1.1.0",
-    "level-sublevel": "^6.3.11",
     "osenv": "~0.1.0",
     "subleveldown": "^4.1.4",
     "tape": "^4.11.0"