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
solution.

Regardless, the cleaning logic is quite redundant now that we run each
test in a hermetic environment. Let's just remove it.