Delete obsolete registry files

[cgay]

1. Add dep strings@1.0 to some project
2. dylan update
3. Remove dep strings from the pkg.json file
4. dylan update
5. There is still a registry entry for strings that points to the strings 1.0 package.

If strings is still being used it should be picked up from the Open Dylan installation or (if it's not in the OD install) the user should get an error.

Conceptually, we should simply remove the entire registry directory before dylan update generates a new one. What I would prefer is to actually show the changes that were made, so just note the files that exist at the start and remove any that don't get overwritten.

[cgay]

That last sentence isn't correct. Only changed registry files are re-written, so really we want to

1. note all registry files at the beginning
2. note all registry files that we considered writing
3. delete anything found in step 1 that isn't in step 2

Is there any need to allow for the possibility that a humanz hand-edited a registry file?