Remove event cache feature flag (#4719)

libraries/featureflag/api/src/main/kotlin/io/element/android/libraries/featureflag/api/FeatureFlags.kt

@@ -147,13 +147,6 @@ enum class FeatureFlags(
         defaultValue = { true },
         isFinished = false,
     ),
-    EventCache(
-        key = "feature.event_cache",
-        title = "Use SDK Event cache",
-        description = "Warning: you must kill and restart the app for the change to take effect.",
-        defaultValue = { true },
-        isFinished = false,
-    ),
     PrintLogsToLogcat(
         key = "feature.print_logs_to_logcat",
         title = "Print logs to logcat",