Use -f argument to rm to force-remove files.

When using pty=true, removing files using sudo may request confirmation,
which will hang the connection.

Similarly, sometimes assumptions about file existence may be wrong and
in those cases it seems better to continue on as long as the file does
not exist, so -f makes sense there, too.
Merge pull request #4410 from lsimons/use-rm-with-f

Use -f argument to rm to force-remove files.