Package.json: remove "private": false field

This field is being removed by Yarn 2+ (`"private": true` stays untouched). This commit basically offloads this PR: adeira/universe#2154

adeira-source-id: 2e06a4b2c37428ed66406781dea2d76e6200fed7

package.json

@@ -9,7 +9,6 @@
     "directory": "src/signed-source"
   },
   "license": "MIT",
-  "private": false,
   "version": "2.0.0",
   "main": "./src/SignedSource.js",
   "type": "commonjs",