Warning deleting mounted files we didn't read or create

[sb10]

Deleting files from a cached writable mount warns about failing when we haven't read or created those files, even though they actually get deleted from s3:

rm -fr .nextflow*
WARN[05-15|12:03:22] Unlink failed                            pkg=muxfys mount=/home/ubuntu/mnt path=/home/ubuntu/.muxfys_cache540925068/cog.sanger.ac.uk/sb10/nextflow/.nextflow/history err="no such file or directory" caller=filesystem.go:912
[...]
EROR[05-15|12:03:22] Rmdir failed                             pkg=muxfys mount=/home/ubuntu/mnt path=/home/ubuntu/.muxfys_cache540925068/cog.sanger.ac.uk/sb10/nextflow/.nextflow/cache/aded2fdd-c1c0-480b-a816-951424f59420/db err="no such file or directory" caller=filesystem.go:758
rm: cannot remove '.nextflow/cache/aded2fdd-c1c0-480b-a816-951424f59420': Function not implemented

The directories remain ls-able, even though they no longer exist.