delete the data-loader package

[tasshi-me]

Since https://github.com/kintone/cli-kintone is published, we can delete the data-loader package from js-sdk.
The data-loader is the development version of https://github.com/kintone/cli-kintone.

We have already made the npm package deprecated.
https://www.npmjs.com/package/@kintone/data-loader

Package no longer supported. Please use https://github.com/kintone/cli-kintone instead.