Remove definition of warnIgnored in CLIEngine options

This flag has no meaning to the `CLIEngine` constructor, it is only
valid as an optional third argument to `executeOnText`. When this was
mistakenly added in #915
this was missed. Luckily the problem it was meant to solve was accepted
as a bug against ESLint itself and fixed there which is why this wasn't
caught earlier.

src/worker-helpers.js

@@ -160,7 +160,6 @@ export function getCLIEngineOptions(type, config, rules, filePath, fileDir, give
   const cliEngineConfig = {
     rules,
     ignore: !config.disableEslintIgnore,
-    warnIgnored: false,
     fix: type === 'fix'
   }