Remove unnecessary Unknown option for HarmBlockThreshold

The value is always specified by the developer and sent to the
backend (not read back), so we don't need  to include that option in
the enum.

Author: rlazo

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

@@ -20,9 +20,6 @@ package com.google.firebase.vertexai.type
  * Represents the threshold for some [HarmCategory] that is allowed and blocked by [SafetySetting].
  */
 enum class HarmBlockThreshold {
-  /** The threshold was not specified. */
-  UNKNOWN,
-
   /** Content with negligible harm is allowed. */
   LOW_AND_ABOVE,