Remove ethpm.json

The package is not registered in ethpm at the moment. This file was an
artifact leftover from vanilla openzeppelin. This PR removes it, until
we decide to register openzeppelin-eth on ethpm.

ethpm.json

-{
-  "package_name": "zeppelin",
-  "version": "2.1.3",
-  "description": "Secure Smart Contract library for Solidity",
-  "authors": [
-    "OpenZeppelin Community <maintainers@openzeppelin.org>"
-  ],
-  "keywords": [
-    "solidity",
-    "ethereum",
-    "smart",
-    "contracts",
-    "security",
-    "zeppelin"
-  ],
-  "license": "MIT"
-}