Update createImagenModel signature

Author: andrewheard

FirebaseVertexAI/Sources/VertexAI+Preview.swift

@@ -17,12 +17,10 @@ import Foundation
 
 @available(iOS 15.0, macOS 12.0, macCatalyst 15.0, tvOS 15.0, watchOS 8.0, *)
 public extension VertexAI {
-  func imagenModel(modelName: String, modelConfig: ImagenModelConfig? = nil,
-                   safetySettings: ImagenSafetySettings? = nil,
+  func imagenModel(modelName: String, safetySettings: ImagenSafetySettings? = nil,
                    requestOptions: RequestOptions = RequestOptions()) -> ImagenModel {
     return createImagenModel(
       modelName: modelName,
-      modelConfig: modelConfig,
       safetySettings: safetySettings,
       requestOptions: requestOptions
     )

FirebaseVertexAI/Sources/VertexAI.swift

@@ -106,8 +106,7 @@ public class VertexAI {
 
   // MARK: - Package APIs
 
-  package func createImagenModel(modelName: String, modelConfig: ImagenModelConfig?,
-                                 safetySettings: ImagenSafetySettings?,
+  package func createImagenModel(modelName: String, safetySettings: ImagenSafetySettings?,
                                  requestOptions: RequestOptions) -> ImagenModel {
     return ImagenModel(
       name: modelResourceName(modelName: modelName),