only bots can use bot challenge decline reason

modules/challenge/src/main/Challenge.scala

@@ -141,6 +141,8 @@ object Challenge {
     val default: DeclineReason = Generic
     val all: List[DeclineReason] =
       List(Generic, Later, TooFast, TooSlow, TimeControl, Rated, Casual, Standard, Variant, NoBot, OnlyBot)
+    val allExceptBot: List[DeclineReason] =
+      all.filterNot(r => r == NoBot || r == OnlyBot)
     def apply(key: String) = all.find { d => d.key == key.toLowerCase || d.trans.key == key } | Generic
   }
 

modules/challenge/src/main/JsonView.scala

@@ -36,7 +36,7 @@ final class JsonView(
       "in"   -> a.in.map(apply(Direction.In.some)),
       "out"  -> a.out.map(apply(Direction.Out.some)),
       "i18n" -> lila.i18n.JsDump.keysToObject(i18nKeys, lang),
-      "reasons" -> JsObject(Challenge.DeclineReason.all.map { r =>
+      "reasons" -> JsObject(Challenge.DeclineReason.allExceptBot.map { r =>
         r.key -> JsString(r.trans.txt())
       })
     )