Root squashing changes the root user to "nfsnobody" who is the owner of the files, so deleting should be possible.
I've been pondering this - it is root on the server that becomes nfsnobody, rood on a client is still root. The idea is that someone with local root access does not get to administer the nfs server.
... discusses this and ways to get around it. I've never seen this on a nas before - presumably it is possible to log in to the server to configure it?
And yes, caution is advised in removing the server root users files - if you don't know how to act as the nas admin, then you should find somebody who does and ask them about it. They are clearly not put there by a guest. There are often good reasons for these files to stay where they are.