Revert "Return early during v6 migration if migration dir exists"

This reverts commit 251f3295f3.

automated install/basic-install.sh

@@ -2146,8 +2146,8 @@ migrate_dnsmasq_configs() {
     # avoid conflicts with other services on this system
 
     # Exit early if this is already Pi-hole v6.0
-    # We decide this on the presence of the migration dir
-    if [[ -d "${V6_CONF_MIGRATION_DIR}" ]]; then
+    # We decide this on the presence of the file /etc/pihole/pihole.toml
+    if [[ -f /etc/pihole/pihole.toml ]]; then
         return 0
     fi