Remove unusable Sentry feature flag

What does this MR do and why?

During our new Sentry rollout we expected to use a feature flag to send events to two Sentry instances.

This was not possible as the client SDK only allows a single instance to be used. The feature flag has been removed to avoid confusion.

Changelog: changed

See more details here: https://gitlab.com/gitlab-com/gl-infra/reliability/-/issues/15372#note_1160691928

I've split this change into different MRs, to make it easier to review:

Remove unusable Sentry feature flag (!104231 - merged)	you are here ☝
Move and rename Sentry constants (!104247 - merged)
Install two versions of Sentry Client SDK (!102790 - merged)	the big one!

Screenshots or screen recordings

NA

How to set up and validate locally

NA

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