Clean up `cleanWebIdeExtensions` logic

MR:

• Clean up `cleanWebIdeExtensions` logic (!415 - merged)
• Remove `web_ide_settings_context_hash` feature ... (gitlab!180261 - merged)

Description

Note: this work is blocked by the following issues:

• Pass extensions marketplace settings hash to We... (gitlab#501286 - closed): this issue completes the implementation of the settings_context_hash
• Enforce not-null constraint on settings_context... (gitlab#503812 - closed): this issue includes performing a backfill to ensure all persisted extensions settings has a settings_context_hash.

Now that the above issues are delivered, we can perform some clean up steps.

Acceptance Criteria

• Remove the web_ide_settings_context_hash feature flag and its usages.
• Remove call to remove settings sync extensions data: this is no longer relevant, as extension settings now exist solely within the settings context hash.
• Modify Example App to not allow null settings context hash if extensions marketplace is enabled.