feature(amd): added some utils to Elgg_Amd_Config class

Utils are unsetPath, unsetShim and removeDependency.
fix(amd): removed elgg_require_js for backwards compatibility

Use elgg_load_js instead. Use elgg_register_js to define sync or async
javascript instead.

`elgg_register_js` manages registrations and loadings of async js better
`elgg_unregister_js` also unregisters async js
`elgg_load_js` also loads async js
Removed `elgg_require_js` function and uses
New private function `_elgg_get_external_file_from_map`

Fixes #6496
test(amd): added tests for AMD module registering and loading