remove application.* assets as manifest files

[johnyb]

These manifest files make more sense in the application itself, not in the engines. The engines should use an index file (see the rails assets pipeline guide for more information on index files) to provide a convenient way to import css and js assets. As a reference, you can have a look into commit ebad8ea63.

Before the switch to index files, a hack was used to get the assets loaded in the application itself. So if all this is supposed to work, the application manifest of the main application might need adjustment. See fassets_core dummy app as an example of how to load the index manifest files of the engines. (Works the same for css)