[ticket/13137] Remove schema.json file

PHPBB3-13137
[ticket/13137] Generate the schema when the schema.json file is not available

PHPBB3-13137
[ticket/13137] Add schema.json file to .gitignore

PHPBB3-13137
[ticket/13137] Improve output of create_schema_files.php

PHPBB3-13137
[ticket/13137] Create schema.json in build script

PHPBB3-13137
Merge pull request #3027 from nickvergessen/ticket/13137

[ticket/13137] Remove schema.json from repository

* nickvergessen/ticket/13137:
  [ticket/13137] Create schema.json in build script
  [ticket/13137] Improve output of create_schema_files.php
  [ticket/13137] Add schema.json file to .gitignore
  [ticket/13137] Generate the schema when the schema.json file is not available
  [ticket/13137] Remove schema.json file