client: drop duplicate SharesChannel re-exports (PR #571 follow-up) (#572)

Mirror of #571 (`Subscription`) applied to `SharesChannel`. Two duplicate
re-exports of the trait at `subscriptions::SharesChannel` are dropped:

- `ibapi::client::SharesChannel` (`src/client/mod.rs:40-41`) — zero
  in-crate consumers.
- `ibapi::client::sync::SharesChannel` (`src/client/sync.rs:452-453`) —
  zero in-crate consumers; was attached to an orphan doc block (talking
  about Subscriptions, not SharesChannel — wrong home anyway).

The orphan doc + duplicated `realtime_bars` example at
`client/sync.rs:419-450` are deleted along with the re-export. The same
example already lives on `RealtimeBarsBuilder` (`market_data/realtime/
sync/mod.rs:31`); the `Subscription`-pattern prose duplicates
`subscriptions/sync.rs:16-35`.

The labelled sync-explicit path `ibapi::client::blocking::SharesChannel`
is preserved — three internal callers (`accounts/sync/mod.rs:5`,
`news/sync.rs:9`, `common/request_helpers.rs:6`) use it.

Net public-API delta:
- Removed: `ibapi::client::SharesChannel`,
  `ibapi::client::sync::SharesChannel`.
- Unchanged: `ibapi::subscriptions::SharesChannel` (canonical),
  `ibapi::client::blocking::SharesChannel` (labelled sync-explicit).

Migration note at `docs/migration-3.0.md` §14. Marks the parent item in
`plans/v3-api-ergonomics.md` §2 as shipped.