Remove unused code and images

[jeffsmale90]

Description

Removes some code and files that are unused.

• Network and token images are now fetched from MetaMask token image repository, so images and iconConstants.ts are removed
• RequestHeader is an unnecessary abstraction, so it's pulled back into permissionHandlerContent.tsx
• isSnapsError was not used, so removed
• iterator is not used, so was removed
• updateAccountsOrder and moveToFirstIndex are not used, so removed

Manual testing steps

Should have no impact.

• I've followed MetaMask 7715 Permissions Snaps Contributor Docs and MetaMask 7715 Permissions Snaps Coding Standards.
• I've completed the PR template to the best of my ability
• I’ve included tests if applicable
• I’ve documented my code using JSDoc format if applicable

Pre-merge reviewer checklist

• I've manually tested the PR (e.g. pull and build branch, run the app, test code being changed).
• I confirm that this PR addresses all acceptance criteria described in the ticket it closes and includes the necessary testing evidence such as recordings and or screenshots.