p2p: delete legacy stack initial pass (#7035)

A few notes:

- this is not all the deletion that we can do, but this is the most
  "simple" case: it leaves in shims, and there's some trivial
  additional cleanup to the transport that can happen but that
  requires writing more code, and I wanted this to be easy to review
  above all else.
  
- This should land *after* we cut the branch for 0.35, but I'm
  anticipating that to happen soon, and I wanted to run this through
  CI.

test/e2e/pkg/manifest.go

@@ -145,9 +145,6 @@ type ManifestNode struct {
 	// This is helpful when debugging a specific problem. This overrides the network
 	// level.
 	LogLevel string `toml:"log_level"`
-
-	// UseLegacyP2P enables use of the legacy p2p layer for this node.
-	UseLegacyP2P bool `toml:"use_legacy_p2p"`
 }
 
 // Stateless reports whether m is a node that does not own state, including light and seed nodes.

test/e2e/pkg/testnet.go

@@ -96,7 +96,6 @@ type Node struct {
 	PersistentPeers  []*Node
 	Perturbations    []Perturbation
 	LogLevel         string
-	UseLegacyP2P     bool
 	QueueType        string
 	HasStarted       bool
 }
@@ -182,7 +181,6 @@ func LoadTestnet(file string) (*Testnet, error) {
 			Perturbations:    []Perturbation{},
 			LogLevel:         manifest.LogLevel,
 			QueueType:        manifest.QueueType,
-			UseLegacyP2P:     nodeManifest.UseLegacyP2P,
 		}
 
 		if node.StartAt == testnet.InitialHeight {