FNAL staging: use -f option for xrdcp

The xrdcp stageout was doing /bin/rm over the FUSE mount; we can just
use the -f option in xrdcp to do the same thing.
FNAL staging: fix string concat error
Merge pull request #5485 from holzman/fnal-xrdcp-force-stageout

FNAL staging: use -f option for xrdcp