Hides expiry date for ads

[NejcZdovc]

Resolves brave/brave-browser#4042

UI implementation: brave/brave-ui#444

Submitter Checklist:

• Submitted a ticket for my issue if one did not already exist.
• Used Github auto-closing keywords in the commit message.
• Added/updated tests for this change (for new code or code which already has tests).
• Verified that these changes build without errors on
  • Windows
  • macOS
  • Linux
• Verified that these changes pass automated tests (npm test brave_unit_tests && npm test brave_browser_tests && npm run test-security) on
  • Windows
  • macOS
  • Linux
• Verified that all lint errors/warnings are resolved (npm run lint)
• Ran git rebase master (if needed).
• Ran git rebase -i to squash commits (if needed).
• Tagged reviewers and labelled the pull request as needed.
• Request a security/privacy review as needed.
• Add appropriate QA labels (QA/Yes or QA/No) to include the closed issue in milestone

Test Plan:

plan 1:

• clean profile
• enable rewards
• claim grant from settings page and panel and make sure that it shows expiration date
• make sure that you also see expiration date in the wallet under details (settings and panel)

plan 2:

• clean profile
• enable rewards
• view some ads
• get ads grant
• make sure that you don't see expiration date on the final step
• make sure that you don't see expiration date in the wallet under details (settings and panel)

Reviewer Checklist:

• New files have MPL-2.0 license header.
• Request a security/privacy review as needed.
• Adequate test coverage exists to prevent regressions
• Verify test plan is specified in PR before merging to source

[tmancey]

nit: same logic is used in a couple places, consider decoupling into a method?

[NejcZdovc]

we only use it here for the settings page, sadly extension code has different util files