Remove redundant null declaration (#6281)

Since the constructor is private, and the builder sets it to null if
missing, this declaration is actually redundant.

b/368368914

Author: rlazo

firebase-vertexai/src/main/kotlin/com/google/firebase/vertexai/type/GenerationConfig.kt

@@ -78,7 +78,7 @@ private constructor(
   val maxOutputTokens: Int?,
   val stopSequences: List<String>?,
   val responseMimeType: String?,
-  val responseSchema: Schema? = null,
+  val responseSchema: Schema?,
 ) {
 
   /**