Remove user legacy Web IDE toggle

• Step 1: Remove user legacy Web IDE toggle (!120063 - merged)
• Step 2: Ignore use_legacy_web_ide column (!120209 - merged)

What does this MR do and why?

Remove user legacy Web IDE toggle. We're getting close to removing the legacy Web IDE altogether (see relevant epic). This is an iterative step towards that.

GitLab instances can still turn off the new Web IDE globally by managing the vscode_web_ide feature flag.

Screenshots or screen recordings

Validate the user preference no longer shows up:

Before	After

Validate that we load the new Web IDE even if the db preference is set:

Before	After

How to set up and validate locally

Validate the user preference no longer shows up:

1. Enable vscode_web_ide feature flag.
2. Visit the User Preference page.
3. The "Web IDE" section should not be present.

Validate that we load the new Web IDE even if the db preference is set:

1. Enable vscode_web_ide feature flag.
2. In a rails console, update a user record with User.find(1).user_preference.update!(use_legacy_web_ide: true).
3. Visit the Web IDE of a project and it should still load the new Web IDE.

MR acceptance checklist

This checklist encourages us to confirm any changes have been analyzed to reduce risks in quality, performance, reliability, security, and maintainability.

• I have evaluated the MR acceptance checklist for this MR.