t5601: rm the target file of cp that could still be executing

t5601: rm the target file of cp that could still be executing

"while sh t5601-clone.sh; do :; done" seems to fail sporadically at
around test #45 where fake-ssh wrapper is copied create plink.exe,
with an error message that says the "text is busy".

I have a mild suspicion that the root cause of the bug is that the
fake SSH process from the previous test is still running by the time
the next test wants to replace it with a new binary, but in the
meantime, removing the target that could still be executing before
copying something else over seems to work it around.

Signed-off-by: Junio C Hamano <gitster@pobox.com>