Fix missing return in WebdavServlet doDelete after precondition failure

[Seth0x41]

When a WebDAV DELETE request includes a conditional header like If-Unmodified-Since or If-Match, and the condition fails, the server correctly responds with HTTP 412 Precondition Failed. However, it still proceeds to delete the resource because doDelete() is missing a return statement after setting the 412 status. The doPut() method handles this correctly by returning immediately.

This means conditional headers are not enforced for DELETE operations. A file can be deleted by any client regardless of the precondition check result.

POC

# 1. Create a file
curl -X PUT -d "protected content" http://localhost:8080/webdav/test.txt

# 2. DELETE with a precondition that should fail (date in the past)
curl -v -X DELETE -H "If-Unmodified-Since: Mon, 01 Jan 2001 00:00:00 GMT" http://localhost:8080/webdav/test.txt
# Returns 412 Precondition Failed

# 3. Check if file still exists
curl -v http://localhost:8080/webdav/test.txt
# Returns 404. File was deleted anyway despite the 412!

The fix adds the missing return after setting the 412 status, consistent with how doPut() already handles this case.