chore: remove core dev dependency, snippet.xml, and system.xml (#5266)

Author: bshaffer

composer.json

@@ -27,7 +27,6 @@
  },
  "require-dev": {
  "phpunit/phpunit": "^4.8|^5.0",
- "google/cloud-core": "^1.31",
  "phpdocumentor/reflection": "^3.0"
  },
  "suggest": {