[DISCO-3262] unconditionally enable jexl

[bendk]

We've audited the jexl-related crates on moz-central, which was the main reason to put this behind a feature flag.

I think we can do this. @gruberb were there any other reasons to disable this? @0c0w3 is this safe to merge now or should we wait until that file size issue is cleared first?

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[0c0w3]

Thanks! This prevents vendoring until we fix the file size problem, but I don't think that should block merging this PR. We can cross that bridge later. I still haven't heard back from #build yet. I'll ping someone individually if I don't hear back soon.