Have replication clients remove _INT_STREAM_POS (#15309)

* Have replication clients remove _INT_STREAM_POS

Suppose worker A makes an internal http request from worker B. B may
make changes that A later learns about over replication. We want A's
request to block until it has seen those changes—mainly to ensure A's
caches are invalidated promptly. This helps provide read-after-write
consistency, eliminating entire categories of races and test flakes.

To implement this, B includes a top-level field `_INT_STREAM_POS` in its
response JSON. Roughly speaking, the field's value tells A what to wait
for. But we weren't removing that internal field before A's request
completed!

Introduced in https://github.com/matrix-org/synapse/pull/14820.
Fixes #15308.

* Changelog

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