feat: Remove blockaid migration BannerAlert

[pedronfigueiredo]

Description

On https://github.com/MetaMask/metamask-extension/pull/23743/, we introduced a BannerAlert that warns the user if they migrated from OpenSea to Blockaid security alerts and they are on a network that doesn't support them.

Since enough time has passed, this PR removes that logic

Related issues

https://github.com/MetaMask/metamask-extension/pull/23743/

Manual testing steps

See instructions on https://github.com/MetaMask/metamask-extension/pull/23743/

Screenshots/Recordings

Before

After

Pre-merge author checklist

• [ ] I've followed MetaMask Contributor Docs and MetaMask Extension 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
• [ ] I’ve applied the right labels on the PR (see labeling guidelines). Not required for external contributors.

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.