Docs: Remove recipe for running shell commands

gulp v4 supports this natively now, tasks can return a child process.

https://gulpjs.com/docs/en/getting-started/async-completion#returning-a-child-process

Author: silvenon

docs/recipes/README.md

@@ -23,4 +23,3 @@
 * [Run Grunt Tasks from Gulp](run-grunt-tasks-from-gulp.md)
 * [Rollup with rollup-stream](rollup-with-rollup-stream.md)
 * [Run gulp task via cron job](cron-task.md)
-* [Running shell commands](running-shell-commands.md)