Retire the etcd volumes schema and host-string parsing helpers

Remove what the migration leaves behind, once nothing reads it any more.

- Drop the etcd volumes configuration schema and the loader that feeds it into the manager configuration.
- Remove the helpers that build and split the proxy and volume names out of the virtual folder host string, along with their call sites. There are roughly fifty of them, so this is the bulk of the work.
- Drop the virtual folder host column and the mount columns on sessions and kernels.

Depends on virtual folder mount resolution reading the database, which is tracked separately: while the manager still asks the storage proxy where a folder lives, the host string cannot be removed.

Design: BEP-1078 (Storage Proxy Enhancement).

JIRA Issue: BA-7694

Start by tracing the etcd volumes configuration schema and loader into manager configuration, then locate the proxy and volume host-string helpers and their call sites. Confirm virtual folder mount resolution reads the database before removing the host and mount columns; done means no remaining reads, helpers, or dependent schema and column definitions.

Written by the indexing model from the issue text.