Revert "feat: Require use of flag to send envelopes (#2594)"

[HazAT]

This reverts commit 0d9725a.

The endpoint works now and we should use it going forward, no option.

[getsentry-bot]

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[rhcarvalho]

Do we need to rush removing this? I'd rather have a quick way out in Sentry in case we run into any problems in the coming weeks, without requiring changes to the SDK.

Then we have time to plan the switch better. There could be more issues than what @dashed found in #2602.

[rhcarvalho]

I'd wait a few more weeks.

[HazAT]

Waiting gives us little benefit, the chances of us uncovering a breaking bugs are low. This almost ran for over a week on sentry.io
We need to send transactions through envelopes sooner rather than later to potentially uncover more. If something new comes up we fix it on the way but I don't see any blocking issue right now even this breadcrumb issue Alberto raised.

Since AM / Tracing is considered a beta feature overall this is fine.

[dashed]

I think it’ll be fine. We’ll be able to surface more issues soon.

[HazAT]

I would go ahead and merge this for the said reasons
cc @rhcarvalho

I'm minority on wanting to wait