Clean up `cleanWebIdeExtensions` logic

Issue: Clean up `cleanWebIdeExtensions` logic (#415 - closed)

What does this MR do and why?

• Remove call to delete extensions settings: Now that we pass the settingsContextHash to the Web IDE, we no longer need to call the settings sync endpoint to remove extensions settings data.
• Modify Example App to pass a default settings context hash based on default extensions marketplace settings. This is done to support the validation logic added in Enforce not-null constraint on settings_context... (gitlab#503812 - closed) to ensure settings_context_hash for extensions settings is not empty. We'll still want the ability to customize the settings context hash value until the mechanisms to allow custom Extensions Marketplace is ready.

MR acceptance checklist

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Screenshots or screen recordings

Default settings context hash	Custom settings context hash
default	custom

How to set up and validate locally

• Run yarn start:example to start the Example App
• Load the Web IDE with Extensions Marketplace enabled and an empty settings context hash field. Note that when sending requests to settings sync endpoints, we're passing a settings context hash based on the default Extensions Marketplace settings (OpenVSX).
• Load the Web IDE with Extensions Marketplace enabled, enter a settings context hash value. Note that the entered settings context hash value is passed when sending requests to settings sync endpoints.