chore(code): Remove HostMsg::ReceivedProposalPart message, use network subscription instead

[romac]

Closes: #XXX

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PR author checklist

For all contributors

• Reference GitHub issue
• Ensure PR title follows the conventional commits spec

For external contributors

• Maintainers of Malachite are allowed to push changes to the branch

[codecov]

❌ 33 Tests Failed:

Tests completed	Failed	Passed	Skipped
155	33	122	0

View the top 3 failed test(s) by shortest run time

informalsystems-malachitebft-test::it n3f0_pubsub_protocol::broadcast_custom_config_2ktx

Stack Traces | 30s run time

No failure message available

informalsystems-malachitebft-test::it n3f0_pubsub_protocol::gossip_custom_config_1ktx

Stack Traces | 30s run time

No failure message available

informalsystems-malachitebft-test::it n3f1::proposer_fails_to_start

Stack Traces | 30s run time

No failure message available

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