Do not show sync warning when creating new chain on desktop

[AlexeyBarabash]

Resolves brave/brave-browser#19314

This PR fixes issue from #10898 - now the warning is not shown when creating a new chain.

Submitter Checklist:

• I confirm that no security/privacy review is needed, or that I have requested one
• There is a ticket for my issue
• Used Github auto-closing keywords in the PR description above
• Wrote a good PR/commit description
• Added appropriate labels (QA/Yes or QA/No; release-notes/include or release-notes/exclude; OS/...) to the associated issue
• Checked the PR locally: npm run test -- brave_browser_tests, npm run test -- brave_unit_tests, npm run lint, npm run gn_check, npm run tslint
• Ran git rebase master (if needed)

Reviewer Checklist:

• A security review is not needed, or a link to one is included in the PR description
• New files have MPL-2.0 license header
• Adequate test coverage exists to prevent regressions
• Major classes, functions and non-trivial code blocks are well-commented
• Changes in component dependencies are properly reflected in gn
• Code follows the style guide
• Test plan is specified in PR before merging

After-merge Checklist:

• The associated issue milestone is set to the smallest version that the
  changes has landed on
• All relevant documentation has been updated, for instance:
  • https://github.com/brave/brave-browser/wiki/Deviations-from-Chromium-(features-we-disable-or-remove)
  • https://github.com/brave/brave-browser/wiki/Proxy-redirected-URLs
  • https://github.com/brave/brave-browser/wiki/Fingerprinting-Protections
  • https://github.com/brave/brave-browser/wiki/Brave%E2%80%99s-Use-of-Referral-Codes
  • https://github.com/brave/brave-browser/wiki/Custom-Headers
  • https://github.com/brave/brave-browser/wiki/Web-Compatibility-Exceptions-in-Brave
  • https://github.com/brave/brave-browser/wiki/QA-Guide
  • https://github.com/brave/brave-browser/wiki/P3A

Test Plan:

I. Don't show warning when creating a new chain

1. Open brave://settings/braveSync/setup
2. Press Start a new Sync Chain
3. Choose Computer
4. Press OK - expected not to see the warning

II. Show warning when connecting to the sync chain

1. Open brave://settings/braveSync/setup
2. Press I have a sync code
3. Paste words and press Confirm - expected the warning is shown

[AlexeyBarabash]

CI failed for reasons not related to the PR:
https://ci.brave.com/job/pr-brave-browser-desktop_sync_warning_fix_for_new_chain-macos/1/execution/node/191/log/
https://ci.brave.com/job/pr-brave-browser-desktop_sync_warning_fix_for_new_chain-linux/1/execution/node/212/log/

for Linux and macOS

[AlexeyBarabash]

Rebased, because CI was keeping fail on the place not from the PR