Fixes bug in grunt definitions

Removes `pkg` property from `IProjectConfig` interface (the config object passed to `grunt.initConfig`) because it is not necessary; it is merely a commonly-used convention.
That's a matter of opinion.  The use of a `pkg` property to store the parsed contents of package.json is merely a convention suggested by the Grunt documentation.  It is not actually used by grunt, and using `pkg` to store something else is allowed.

The type information is technically the same either way since `[plugin: string]: any` covers it.  If someone wanted to write a doc comment for `pkg` they could re-add it and write the appropriate documentation.  I didn't feel the need to do so, but I'm not particularly attached to either option.