package: Finish removing deprecated postinstall-build package.

Commit 5821143 removed the
configuration but not the package itself.  Really fixes #72.

Signed-off-by: Anders Kaseorg <[email protected]>

Author: andersk

package-lock.json

@@ -15,8 +15,7 @@
     "fs-readfile-promise": "^3.0.1",
     "ini": "^1.3.5",
     "isomorphic-fetch": "^2.2.1",
-    "isomorphic-form-data": "2.0.0",
-    "postinstall-build": "^5.0.3"
+    "isomorphic-form-data": "2.0.0"
   },
   "devDependencies": {
     "@babel/cli": "^7.6.4",