Bug description
On staging.openhistoricalmap.org, where we deployed the latest iD changes with automatically incrementing Source X fieldsets, there's a weird offset of the delete behavior.

Here's a schematic of what is happening:

It looks to me like the Source fieldsets get created as Source, Source 1, Source 2,... but the trashcans are pointed at Source 1, Source 2...

What should be happening?
Since the base Source fieldset in the preset can never be deleted, it should never get a trashcan. Trashcans should appear only starting with Source 1 and the /trashcan on a fieldset should delete that fieldset, not the next one down.

Repro Steps
Please provide detailed instructions to reproduce the behavior:

1. Go to https://staging.openhistoricalmap.org/edit
2. Enter any string in the Source field
3. Use the Add Field menu to add Source 1
4. Use the trashcan on Source to delete Source 1