insights: remove graphql clients from backend

Created by: leonore

closes #37968 (closed). wehee!

big PR so points to note:

• site config option has been marked as deprecated
• removed some delegate functions in the worker
• pulled some common logic into the worker Handle functions (erroring early on JIT, persisting recordings - means adding any future paths we don't forget anything)
• remove aaaallll (read: most) the graphql files in our executors! wahey! | | |

follow-up issues:

https://github.com/sourcegraph/sourcegraph/issues/38471 https://github.com/sourcegraph/sourcegraph/issues/38472

Test plan

Ran existing unit tests. Built sg start enterprise-codeinsights Created queries/monitored things on local dev instance, but everything that's been removed is stuff that shouldn't be hit anymore.