Drop root_subvolume_read_only documentation

doc/control-file.md

@@ -923,12 +923,6 @@ should be an extra warning pop-up dialog when the user enters the expert
 partitioner dialog during installation, for example because the product has
 special requirements for partitioning (Btrfs to support snapshots etc.).
 
-*root_subvolume_read_only* (boolean, default _false_) specifies whether the
-root subvolume should be mounted read-only in /etc/fstab and its 'ro' Btrfs
-property should be set to _true_. This works only for Btrfs root
-filesystems. If another root filesystem type is chosen, this might fail
-silently.
-
 *home_path* (string) is the path (mount point) for the home
 partition or volume, if any is created (depending on *try_separate_home*,
 *limit_try_home* and available disk space).