fix(storage): Unlink symlink instead of deleting target content and link

Nextcloud allow following symlink (https://docs.nextcloud.com/server/latest/admin_manual/configuration_server/config_sample_php_parameters.html#localstorage-allowsymlinks) but removal of symlink remove files in the target of the symlink and fail to remove the link returning an error.

Using the following structure:

```
.
├── afolder
│   └── test.txt
├── alink -> afolder
└── welcome.txt
```

created with:

```bash
mkdir afolder
touch afolder/test.txt
ln -s afolder alink
```

After deletion of `alink` symbolic link, the content of `afolder` is removed, the link also and an error is reported.

```json
{
  "method":"DELETE",
  "url":"/remote.php/dav/files/admin/alink",
  "message":"rmdir(/data/dev/nextcloud/data/admin/files/alink): Not a directory at /data/dev/nextcloud/lib/private/Files/Storage/Local.php#128"
}
```

Results is:
```
.
├── afolder
└── welcome.txt
```

If the resource to be deleted is a link, unlink it (and preserve link target content).

```
.
├── afolder
│   └── test.txt
└── welcome.txt
```

Signed-off-by: Francois Prunayre <fx.prunayre@gmail.com>

Author: fxprunayre

lib/private/Files/Storage/Local.php

@@ -100,35 +100,40 @@ public function rmdir(string $path): bool {
 			return false;
 		}
 		try {
-			$it = new \RecursiveIteratorIterator(
-				new \RecursiveDirectoryIterator($this->getSourcePath($path)),
-				\RecursiveIteratorIterator::CHILD_FIRST
-			);
-			/**
-			 * RecursiveDirectoryIterator on an NFS path isn't iterable with foreach
-			 * This bug is fixed in PHP 5.5.9 or before
-			 * See #8376
-			 */
-			$it->rewind();
-			while ($it->valid()) {
+			if (is_link($this->getSourcePath($path))) {
+				clearstatcache(true, $this->getSourcePath($path));
+				return unlink($this->getSourcePath($path));
+			} else {
+				$it = new \RecursiveIteratorIterator(
+					new \RecursiveDirectoryIterator($this->getSourcePath($path)),
+					\RecursiveIteratorIterator::CHILD_FIRST
+				);
 				/**
-				 * @var \SplFileInfo $file
+				 * RecursiveDirectoryIterator on an NFS path isn't iterable with foreach
+				 * This bug is fixed in PHP 5.5.9 or before
+				 * See #8376
 				 */
-				$file = $it->current();
-				clearstatcache(true, $file->getRealPath());
-				if (in_array($file->getBasename(), ['.', '..'])) {
+				$it->rewind();
+				while ($it->valid()) {
+					/**
+					 * @var \SplFileInfo $file
+					 */
+					$file = $it->current();
+					clearstatcache(true, $file->getRealPath());
+					if (in_array($file->getBasename(), ['.', '..'])) {
+						$it->next();
+						continue;
+					} elseif ($file->isFile() || $file->isLink()) {
+						unlink($file->getPathname());
+					} elseif ($file->isDir()) {
+						rmdir($file->getPathname());
+					}
 					$it->next();
-					continue;
-				} elseif ($file->isFile() || $file->isLink()) {
-					unlink($file->getPathname());
-				} elseif ($file->isDir()) {
-					rmdir($file->getPathname());
 				}
-				$it->next();
+				unset($it);  // Release iterator and thereby its potential directory lock (e.g. in case of VirtualBox shared folders)
+				clearstatcache(true, $this->getSourcePath($path));
+				return rmdir($this->getSourcePath($path));
 			}
-			unset($it);  // Release iterator and thereby its potential directory lock (e.g. in case of VirtualBox shared folders)
-			clearstatcache(true, $this->getSourcePath($path));
-			return rmdir($this->getSourcePath($path));
 		} catch (\UnexpectedValueException $e) {
 			return false;
 		}