(core) remove deprecated /download endpoint in favor of newer /api/docs/NNNN/download

Summary:
This endpoint has started to fail when called between a pair
of doc workers. The simplest fix is to simply remove it, it serves no
purpose.

Test Plan: added basic deployment test

Reviewers: dsagal

Reviewed By: dsagal

Subscribers: dsagal

Differential Revision: https://phab.getgrist.com/D3636

test/server/lib/DocApi.ts

@@ -1767,6 +1767,13 @@ function testDocApi() {
     assert.notMatch(resp.data, /grist_Tables_column/);
   });
 
+  // A tiny test that /copy doesn't throw.
+  it("POST /docs/{did}/copy succeeds", async function() {
+    const docId = docIds.TestDoc;
+    const worker1 = await userApi.getWorkerAPI(docId);
+    await worker1.copyDoc(docId, undefined, 'copy');
+  });
+
   it("GET /docs/{did}/download/csv serves CSV-encoded document", async function() {
     const resp = await axios.get(`${serverUrl}/api/docs/${docIds.Timesheets}/download/csv?tableId=Table1`, chimpy);
     assert.equal(resp.status, 200);