Build: move list of ignored JS files from the Grunt configuration into the `.jshintignore` file.

The list of ignored JS files in the Grunt configuration was only used by the jshint task, so let's move these into the recently-introduced (r9146) jshint ignore file.

See #6015

git-svn-id: https://buddypress.svn.wordpress.org/trunk@9148 cdf35c40-ae34-48e0-9cc9-0c9da1808c22