Remove the top level --boot flag from ResumeActorRequest (#1548)

Fixes https://github.com/agent-substrate/substrate/issues/1566

Today the Resume workflow resolves its restore source in the following
order
1. First check if the actor has node-local snapshot, 
2. then its own durable external snapshot, 
3. then the template's golden snapshot. 
 
The boot flag was consulted at exactly one point in that chain, where it
suppressed using the golden-snapshot, which made its behavior much
narrower than "boot from scratch" suggests:

- Actor has its own external snapshot and boot=true: flag ignored,
restores the actor's snapshot.
- Actor has a local snapshot and boot=true: flag ignored, restores the
local snapshot.
- Actor has no snapshot, template has no golden snapshot: cold boot from
the spec regardless of the boot flag.
- Actor has no snapshot, template has a golden snapshot: boot=false
restores the golden, boot=true cold boots from the spec. This is the
only case where the flag is used.


The glutton benchmark was the only caller that set boot=true, on each
actor's first resume, to report true cold-start latency as a separate
ResumeActorColdStart stats row. @maxsmythe let me know if this is
required.

The proto field number and name were reserved.

- [x] Tests pass
- [x] Appropriate changes to documentation are included in the PR

docs/api-guide.md

@@ -477,7 +477,6 @@ Because the guards are required and only a read supplies them, an update is alwa
 Activates a suspended actor by restoring it onto a physical worker.
 *   **Request:** `ResumeActorRequest`
     *   `actor`: `ObjectRef` of the actor to resume.
-    *   `boot`: (Optional) If `true`, bypasses snapshots and performs a cold boot.
 *   **Response:** `ResumeActorResponse` containing the updated `Actor` object (including the physical worker placement in `status.worker_assignment`).
 
 #### `SuspendActor`