Reverts logic for unloading units in askLoadedType

The loagic for handling loading/unloading of units in `askLoadedType` was moved
in the compiler side. See https://github.com/scala/scala/pull/3209.  (Also,
mind that this commit should be merged together with the previously linked
scala PR).

This reverts most of the changes made in SHA:
43f8f2bdf4bd99cc85da7f54a0f374be737d4b2f