indexer-agent: Leave subgraph deployment assignment decisions to graph-node

[fordN]

Hands off the assignments

Resolve #853

Changes

• Use recently added subgraph_pause method to explicitly pause subgraph deployments rather than virtually pausing them by assigning to a non-existent index node
• Remove index-node-ids startup parameter from indexer-agent
• Never send an index-node-id param with subgraph_deploy requests, the receiving graph-node will make the index-node-id decision based on the graph-node.toml config file

[fordN]

I wonder if we should include a migration script with this PR to reassign (to existing node id) and pause any subgraph deployments that are currently assigned to node_id = "removed".
@incrypto32 when upgrading the graph-node version to one that supports the subgraph_pause method is there already a data migration?

[incrypto32]

@fordN no there is no migration