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

@@ -15,7 +15,8 @@ import org.matrix.rustcomponents.sdk.JoinRule as RustJoinRule
 fun RustJoinRule.map(): JoinRule {
     return when (this) {
         RustJoinRule.Public -> JoinRule.Public
-        RustJoinRule.Private -> JoinRule.Private
+        // Assume a private join rule is invite only instead. Private shouldn't be in use in Matrix.
+        RustJoinRule.Private -> JoinRule.Invite
         RustJoinRule.Knock -> JoinRule.Knock
         RustJoinRule.Invite -> JoinRule.Invite
         is RustJoinRule.Restricted -> JoinRule.Restricted(rules.map { it.map() }.toImmutableList())
@@ -27,7 +28,6 @@ fun RustJoinRule.map(): JoinRule {
 fun JoinRule.map(): RustJoinRule {
     return when (this) {
         JoinRule.Public -> RustJoinRule.Public
-        JoinRule.Private -> RustJoinRule.Private
         JoinRule.Knock -> RustJoinRule.Knock
         JoinRule.Invite -> RustJoinRule.Invite
         is JoinRule.Restricted -> RustJoinRule.Restricted(rules.map { it.map() })