services/horizon: remove captive-core-use-db parameter

[mollykarcher]

What problem does your feature solve?

Core team wants to remove support for in-memory mode. Core's -in-memory mode proxy's directing to our captive-core-use-db config param, which is currently deprecated, but still exists.

What would you like to see?

• Remove the captive-core-use-db flag. Behavior should now be forced into "true" for this flag.
• Remove anywhere else that we may pass the --in-memory flag to captive core
• Remove the flag from any internal/external documentation
• Update puppet deployments to remove the flag
• Update k8s manifests/helm charts to remove the flag

Note that this is technically a breaking change so it should align with a protocol boundary. Core wants to make their corresponding breaking change in a minor version release. Ideally, this should wait for protocol 23, but pending outcome of slack discussion here if Core will defer their change.

Related:

• services/horizon: change default for --captive-core-use-db to true #4856
• services/horizon: deprecate in-memory mode #5203