fix: Remove references to `gl.addAccount` command

Description

The command was removed but referenced in the extension Walkthrough as well as in the info message

Related Issues

Resolves #1506 (closed)

How has this been tested?

Notification message

1. Remove all GitLab accounts from the extension
2. In the command palette click GitLab: Validate accounts
3. There should be no button Add account

Walkthouth

1. Run the Help: Welcome command
2. choose "How to use GitLab workflow"
3. The first step should contain the actionable link "authenticate to GitLab"

Screenshots (if appropriate)

What CHANGELOG entry will this MR create?

• fix: Bug fix fixes - a user-facing issue in production - included in changelog
• feature: New feature - a user-facing change which adds functionality - included in changelog
• BREAKING CHANGE: (fix or feature that would cause existing functionality to change) - should bump major version, mentioned in the changelog
• None - other non-user-facing changes

Closes #1506 (closed)