Remove JoinRule.Private from the codebase (#6129)

It shouldn't be in use, since it was never properly defined:

- The `Private` values coming from the SDK are considered to be `Invite` instead.
- Remove any existing `JoinRule.Private` usages in our code.

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/media/MediaPreviewValue.kt

@@ -37,7 +37,6 @@ fun MediaPreviewValue?.isPreviewEnabled(joinRule: JoinRule?): Boolean {
         null, On -> true
         Off -> false
         Private -> when (joinRule) {
-            is JoinRule.Private,
             is JoinRule.Knock,
             is JoinRule.Invite,
             is JoinRule.Restricted,

libraries/matrix/api/src/main/kotlin/io/element/android/libraries/matrix/api/room/join/JoinRule.kt

@@ -14,7 +14,6 @@ import kotlinx.collections.immutable.ImmutableList
 @Immutable
 sealed interface JoinRule {
     data object Public : JoinRule
-    data object Private : JoinRule
     data object Knock : JoinRule
     data object Invite : JoinRule
     data class Restricted(val rules: ImmutableList<AllowRule>) : JoinRule