Remove `web_ide_settings_context_hash` feature flag

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

What does this MR do and why?

Now that we've tested the experience of users migrating existing extensions to settings context hash implementation in production, we can remove the web_ide_settings_context_hash feature flag.

Note: the issues states that the backfill is blocking this change, but I'm changing the order of operations. Removing the flag will ensure that the settings context hash is always passed in. That way, we can be sure that we will backfill all null extensions settings settings_context_hash.

References

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• Clean up `cleanWebIdeExtensions` logic (gitlab-web-ide#415 - closed)

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Screenshots or screen recordings

demo

How to set up and validate locally

1. Load the Web IDE
2. Ensure that settings_sync requests are made with settings context hash passed in
3. To simulate the experience for users first time encountering settings context hash:
   1. Remove web_ide_last_extensions_marketplace_settings_context_hash from local storage
   2. Have extensions installed
   3. Reload the Web IDE, extensions should be preserved