Remove unnecessary change to system_custom_theme()

From #1463656-100: Add a Drupal kernel; leverage HttpFoundation and HttpKernel:

Additionally, we should remove all unnecessary changes from this branch/patch.

One such unnecessary change is changing from current_path() to $request->attributes->get('system_path') from within system_custom_theme().

From #1463656-122: Add a Drupal kernel; leverage HttpFoundation and HttpKernel:

Converting to $request->attributes->get('system_path'): I believe that was done before the index.php patch, at which point current_path() was still not actually useful. I'm fine with leaving it as an example. That code is going to change eventually anyway, so I don't know how much work we want to put into it either direction.

While sometimes I think having an example for important API changes is essential (e.g., the initial CMI patch included the porting of image styles to use it), in this case, I think it isn't needed, and creates clutter for reviewers. I'm not even sure we're fully settled on the exact syntax we'll want for changing current_path(), so I think it'll be better for us to do all current_path() replacements together in a follow-up issue rather than include an example in the initial patch.

So, here's a patch to revert that.