testsuite: Remove redundant cleaning logic from T16511
The GHCi script for T16511 had some rm commands to clean up output
from previous runs. This should be harmless since stderr was redirected
to /dev/null; however, it seems that this redirection doesn't work on
Windows (perhaps because GHCi uses cmd to execute the command-line;
I'm not sure). I tried to fix it but was unable to find a sensible
Regardless, the cleaning logic is quite redundant now that we run each
test in a hermetic environment. Let's just remove it.