Remove unused strings helpers

For future use of these helpers, we should use `utils/StringUtils.js` from Brackets.

Removes String.endsWith and String.format: we don't use them anywhere.

Util method lines for Brackets: [`StringUtils#format`](https://github.com/adobe/brackets/blob/master/src/utils/StringUtils.js#L46) and [`StringUtils#endsWith`](https://github.com/adobe/brackets/blob/master/src/utils/StringUtils.js#L129).