Remove scripted-checks feature flag

[ckbedwell]

Problem

Scripted checks in SM have been GA for a while now but live behind a feature flag. For our Cloud customers this isn't a problem as the flag is enabled by default but for on-prem and other developers who are running the plugin locally or in their own Grafana instance it can be confusing why scripted checks are unavailable and unclear how to fix.

Acceptance criteria

• Remove scripted-checks feature flag and ensure scripted checks are still present to all users
• Please modify and/or add the appropriate integration tests