Merge reworked #13690: timezone-derived region and channel mirror prepends

Drop the .cn servers.sed twins, the USTC Arch fallback and the closed
#14399 timezone preselection. region.sh follows the installer's marker.

migrations/1788112314.sh

@@ -7,6 +7,6 @@ echo "Point rc-channel installs at the rc package repository"
 # shipped pairing: an administrator who chose another combination keeps it.
 if grep -q "https://rc-mirror.omarchy.org/" /etc/pacman.d/mirrorlist &&
   grep -q "^Server = https://pkgs.omarchy.org/edge/" /etc/pacman.conf; then
-  sudo sed -i -E "s#^Server = https://pkgs\.omarchy\.(org|cn)/edge/#Server = https://pkgs.omarchy.\1/rc/#" /etc/pacman.conf
+  sudo sed -i "s|^Server = https://pkgs.omarchy.org/edge/|Server = https://pkgs.omarchy.org/rc/|" /etc/pacman.conf
   echo "Switched the [omarchy] repository to the rc channel to match this machine's rc mirror."
 fi

migrations/1791387144.sh

@@ -1,12 +0,0 @@
-echo "Rename the China region to cn and follow its moved profile"
-
-[[ $(cat /etc/omarchy/region 2>/dev/null) == "china" ]] || exit 0
-
-# Links first, so a failure leaves china for the retry.
-for link in /etc/environment.d/90-omarchy-china.conf /.cargo/config.toml /etc/gemrc; do
-  old=$(readlink "$link" || true)
-  if [[ $old == */default/region/china/* ]]; then
-    sudo ln -sfn "$OMARCHY_PATH/default/regions/cn/${old##*/}" "$link"
-  fi
-done
-echo cn | sudo tee /etc/omarchy/region >/dev/null