Removed obsolete classes from buttons in preferences.js

[luixxiul]

• 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).
• Ran git rebase -i to squash commits (if needed).

".wideButton" was removed from addfunds, copyButton, advancedSettings as their width is specified exclusively

Auditors: @bsclifton

Test Plan:

1. Open about:preferences#payments
2. Make sure the width of "Add funds..." and "Advanced Settings..." button is not affected with the change
3. Click "Advanced Settings..."
4. Click "Backup your wallet"
5. Make sure the width of two "Copy" buttons is not affected with the change

[luixxiul]

obviously .wideButton is not a popular className; after this commit is merged it will be only two buttons on .bookmarkHanger that the className is added to.

[bsclifton]

LGTM 👍