Remove min-width for OK button on payment history dialog

[luixxiul]

Describe the issue you encountered:

Because the min-width for the buttons were specified with #6384 globally, the min-width for the OK button on the payment history dialog should be removed.

Expected behavior:

• Brave Version (revision SHA): 0.13.0 Preview8

• Steps to reproduce:

  1. Disable the ledger
  2. run npm run add-simulated-payment-history some times
  3. Enable the ledger
  4. Click "View Payment History..."

• Any related issues: Setting min-width for buttons (l10n-friendly) #6384 OK button should be larger on Payment History dialog #6232 Refactor about:preferences#payments followups #6047

Test Plan:

1. Disable the ledger
2. run npm run add-simulated-payment-history some times
3. Enable the ledger
4. Click "View Payment History..."
5. Make sure the min-width of "OK" button is set to 78px