Revert "llm: add service.ModelName"

Turns out I don't need this.

This reverts commit 022b3638c2d2691d60f430d8984959c91eee46ab.
diff --git a/llm/ant/ant.go b/llm/ant/ant.go
index efebf26..d1c366a 100644
--- a/llm/ant/ant.go
+++ b/llm/ant/ant.go
@@ -41,10 +41,6 @@
 	MaxTokens int          // defaults to DefaultMaxTokens if zero
 }
 
-func (s *Service) ModelName() string {
-	return cmp.Or(s.Model, DefaultModel)
-}
-
 var _ llm.Service = (*Service)(nil)
 
 type content struct {
diff --git a/llm/gem/gem.go b/llm/gem/gem.go
index 5f01db7..6686058 100644
--- a/llm/gem/gem.go
+++ b/llm/gem/gem.go
@@ -31,10 +31,6 @@
 
 var _ llm.Service = (*Service)(nil)
 
-func (s *Service) ModelName() string {
-	return cmp.Or(s.Model, DefaultModel)
-}
-
 // These maps convert between Sketch's llm package and Gemini API formats
 var fromLLMRole = map[llm.MessageRole]string{
 	llm.MessageRoleAssistant: "model",
diff --git a/llm/llm.go b/llm/llm.go
index ba6bd03..1e53ea3 100644
--- a/llm/llm.go
+++ b/llm/llm.go
@@ -13,8 +13,6 @@
 type Service interface {
 	// Do sends a request to an LLM.
 	Do(context.Context, *Request) (*Response, error)
-
-	ModelName() string
 }
 
 // MustSchema validates that schema is a valid JSON schema and returns it as a json.RawMessage.
diff --git a/llm/oai/oai.go b/llm/oai/oai.go
index 4b046b0..8b64157 100644
--- a/llm/oai/oai.go
+++ b/llm/oai/oai.go
@@ -225,10 +225,6 @@
 
 var _ llm.Service = (*Service)(nil)
 
-func (s *Service) ModelName() string {
-	return s.Model.UserName
-}
-
 // ModelsRegistry is a registry of all known models with their user-friendly names.
 var ModelsRegistry = []Model{
 	GPT41,