common,cli,agent: remove multi-network-mode

[dwerner]

This PR removes multi-network mode while preserving the network config parsing logic that it was tied to. This enables a significant reduction in complexity.

Changes:

• Now the agent expects there to only be a single yaml file in the network-specifications-dir
• If network-specifications-dir is provided then the values are used, otherwise single-network mode's style of pulling values from argv/env are used.
• Transfer (L1 -> L2) code is removed since the transfer tool is removed now.
• the env var INDEXER_AGENT_MULTINETWORK_MODE is ignored