refactor(client,curriculum): remove showSpeakingButton from challenge metadata (#66408)

curriculum/schema/__snapshots__/challenge-schema.test.mjs.snap

@@ -2015,9 +2015,6 @@ exports[`challenge schema > should not be changed without informing the mobile t
       },
       "type": "object",
     },
-    "showSpeakingButton": {
-      "type": "boolean",
-    },
     "solutions": {
       "items": [
         {

curriculum/schema/challenge-schema.js

@@ -359,7 +359,6 @@ export const schema = Joi.object().keys({
         'array.unique': 'Dialogues must not have overlapping times.'
       })
   }),
-  showSpeakingButton: Joi.bool(),
   // This is only to be used for dynamic client updates.
   sourceLocation: Joi.string(),
   solutions: Joi.array().items(Joi.array().items(fileJoi).min(1)),