remove: drop support for claude-2 and claude-1 series models

- Remove claude-instant-1.2, claude-2, claude-2.0, claude-2.1 from model lists
- Remove /v1/complete endpoint support (legacy completion API)
- Remove RequestModeCompletion and related code paths
- Simplify handler functions by removing requestMode parameter
- Update all channel adaptors that referenced claude handlers

relay/channel/claude/constants.go

@@ -1,10 +1,6 @@
 package claude
 
 var ModelList = []string{
-	"claude-instant-1.2",
-	"claude-2",
-	"claude-2.0",
-	"claude-2.1",
 	"claude-3-sonnet-20240229",
 	"claude-3-opus-20240229",
 	"claude-3-haiku-20240307",