Update PushProvider API, remove isAvailable(), but instead rely on getDistributors() to eventually return an empty list of Distributors.

libraries/push/api/src/main/kotlin/io/element/android/libraries/push/api/PushService.kt

@@ -29,8 +29,7 @@ interface PushService {
     suspend fun getCurrentPushProvider(): PushProvider?
 
     /**
-     * Return the list of push providers, available at compile time, and
-     * available at runtime, sorted by index.
+     * Return the list of push providers, available at compile time, sorted by index.
      */
     fun getAvailablePushProviders(): List<PushProvider>