Remove Element Call feature flag and revert base URL to call.element.io (#1809)

Co-authored-by: ElementBot <benoitm+elementbot@element.io>

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

@@ -55,12 +55,6 @@ enum class FeatureFlags(
         description = "Allow user to lock/unlock the app with a pin code or biometrics",
         defaultValue = true,
     ),
-    InRoomCalls(
-        key = "feature.elementcall",
-        title = "Element call in rooms",
-        description = "Allow user to start or join a call in a room",
-        defaultValue = true,
-    ),
     Mentions(
         key = "feature.mentions",
         title = "Mentions",