Enable detekt rule UnusedPrivateMember and cleanup the code.

services/analyticsproviders/posthog/src/main/kotlin/io/element/android/services/analyticsproviders/posthog/PosthogAnalyticsProvider.kt

@@ -101,9 +101,11 @@ class PosthogAnalyticsProvider @Inject constructor(
      * We avoid sending nulls as part of the UserProperties as this will reset the values across all devices.
      * The UserProperties event has nullable properties to allow for clients to opt in.
      */
+    /*
     private fun Map<String, Any?>.toPostHogUserProperties(): Properties {
         return Properties().apply {
             putAll(this@toPostHogUserProperties.filter { it.value != null })
         }
     }
+     */
 }