(core) Remove code for unused welcome flows

Summary: Removes code that was marked for removal.

Test Plan: Existing tests still pass.

Reviewers: paulfitz

Reviewed By: paulfitz

Differential Revision: https://phab.getgrist.com/D3289

test/nbrowser/Smoke.ts

@@ -14,10 +14,6 @@ async function openMainPage() {
   await driver.get(`${server.getHost()}`);
   while (true) {    // eslint-disable-line no-constant-condition
     try {
-      const url = await driver.getCurrentUrl();
-      if (url.match(/welcome\//)) {
-        await driver.findContent('button', /Continue/).click();
-      }
       if (await driver.findContent('button', /Create Empty Document/).isPresent()) {
         return;
       }