Revert "move config.assets.precompile into config/application.rb"

This reverts commit 304f4d4c25ccabdbf97d37dd7a92a54d0b63a9c9.

As it turns out everything should behave as it did and we don't
need to move this configuration option.
Merge pull request #9398 from senny/revert_precompile_config_move

Revert "move config.assets.precompile into config/application.rb"