Revert changes to Load that have broken the LoadTest system test

This reverts the changes to Load from commits
  - 67348e40e93910e7b3d1db0c3f7cefc5942b9d76
  - cf334a95bbd34359b704644f469757920814432b

We need a better way for a specific loader to request that it will handle
all files that Load has found.
Refs #10061