Worker heartbeat: Remove heartbeat from non-nexus polling (#633)

_**READ BEFORE MERGING:** All PRs require approval by both Server AND
SDK teams before merging! This is why the number of required approvals
is "2" and not "1"--two reviewers from the same team is NOT sufficient.
If your PR is not approved by someone in BOTH teams, it may be summarily
reverted._

<!-- Describe what has changed in this PR -->
**What changed?**
Removed the `worker_heartbeat` field from non-nexus calls

<!-- Tell your future self why have you made these changes -->
**Why?**
We only want `worker_heartbeat` info sent on nexus polls, worker
shutdown, and dedicated worker heartbeat requests.

<!-- Are there any breaking changes on binary or code level? -->
**Breaking changes**
Breaking change for a WIP feature, should have no effect. Also tried
grepping for this field in server and didn't find it used anywhere.

<!-- If this breaks the Server, please provide the Server PR to merge
right after this PR was merged. -->
**Server PR**

Author: yuandrew