🗑️ Remove WebDAV sync support; SuperSync only 🗑️

WebDAV was added as an alternative sync provider but the design decision
is to ship SuperSync only. Removes all WebDAV env vars, the nginx reverse
proxy block for /webdav/, and the WebDAV branch in __pre_execute that
wrote sync-config-default-override.json. SuperSync behavior is unchanged.
- Dockerfile: drop WEBDAV_BACKEND, WEBDAV_BASE_URL, WEBDAV_USERNAME,
WEBDAV_SYNC_FOLDER_PATH from ENV declarations
- rootfs/usr/local/etc/docker/init.d/zz-nginx.sh: remove WEBDAV_*
variable declarations, nginx /webdav/ proxy location block, and
WebDAV branch in __pre_execute sync config generation

Dockerfile
rootfs/usr/local/etc/docker/functions/entrypoint.sh
rootfs/usr/local/etc/docker/init.d/zz-nginx.sh

Dockerfile

@@ -117,10 +117,6 @@ ENV ENV="~/.bashrc" \
     HOSTNAME="casjaysdev-${IMAGE_NAME}" \
     WWW_ROOT_DIR="${WWW_ROOT_DIR}" \
     APP_PORT="80" \
-    WEBDAV_BACKEND="" \
-    WEBDAV_BASE_URL="" \
-    WEBDAV_USERNAME="" \
-    WEBDAV_SYNC_FOLDER_PATH="" \
     SYNC_INTERVAL="" \
     IS_COMPRESSION_ENABLED="" \
     IS_ENCRYPTION_ENABLED="" \