Remove complex cost model support

[Theodus]

As of release v24.2.0, the gateway no longer supports "complex" cost models. The gateway only supports cost models of the form default => x;, where x is a literal value. To avoid user confusion and reduce maintenance costs, I suggest the following changes:

• Remove the DAI injection feature. Based on a review of cost models from active indexers, this feature is not in use.
• Remove the indexer cost set/get variables commands from the CLI
• Replace the indexer cost set/get model with indexer cost set/get, where the user can only set a float value. At this point the @graphprotocol/cost-model dependency can be removed.