I'm tracking down a problem with the perl package that's been
generating this error:
```
OSError: OSError: [Errno 2] No such file or directory: '/blah/blah/blah/lib/5.24.1/x86_64-linux/Config.pm~'
```
The real problem is upstream, but it's being masked by an exception
raised in `filter_file`s finally block.
In my case, `backup` is `False`.
The backup is created around line 127, the `re.sub()` calls
fails (working on that), the `except` block fires and moves the backup
file back, then the finally block tries to remove the non-existent
backup file.
This change just avoids trying to remove the non-existent file.
464e558aea