Remove dbt_experimental_sample_mode environment variable gate

[QMalcolm]

Housekeeping

• I am a maintainer of dbt-core

Short description

Once "sample mode" is relatively stable, we should remove the environment variable dbt_experimental_sample_mode gating of the functionality. We should do this either as part of the beta, but at the latest needs to happen before the GA of dbt-core 1.10.0.

Acceptance criteria

• The environment variable gating for sample mode has been removed

Suggested Tests

• the environment variable dbt_experimental_sample_mode has no effect on one's ability to run in "sample_mode"

Impact to Other Teams

N/A

Will backports be required?

No, it's a new feature

Context

#11227