Remove 'preinstall'

This is an obsolete workaround for old npm versions that broke with bundling but also did not install deps predictably.

Refs:

 - https://github.com/mapbox/node-pre-gyp/blob/master/README.md#configuring
 - mapbox/node-pre-gyp#260

package.json

@@ -39,7 +39,6 @@
     "node": ">=6"
   },
   "scripts": {
-    "preinstall": "npm install node-pre-gyp",
     "install": "node-pre-gyp install --fallback-to-build=false || make node",
     "test": "tape platform/node/test/js/**/*.test.js",
     "test-memory": "node --expose-gc platform/node/test/memory.test.js",