(core) Make Attachments columns get treated like RefLists more

Summary:
Treat the column type 'Attachments' as equivalent to 'RefList:_grist_Attachments' in a few places, because that's essentially what it is. The main goal was to fix parsing strings representing attachments (reflists).

Also removed an unused function.

Test Plan: Tested manually that pasting a CSV/JSON string representation of an attachments reflists works now.

Reviewers: paulfitz

Reviewed By: paulfitz

Subscribers: paulfitz

Differential Revision: https://phab.getgrist.com/D3338

app/common/ValueParser.ts

@@ -220,6 +220,7 @@ export const valueParserClasses: { [type: string]: typeof ValueParser } = {
   ChoiceList: ChoiceListParser,
   Ref: ReferenceParser,
   RefList: ReferenceListParser,
+  Attachments: ReferenceListParser,
 };
 
 /**