(maint) Remove unused files and dirs under ext during packaging

Currently, razor debian builds are failing because rmdir is called
on ext during the dh_install in the debian/rules, but ext still has
contents. This commit adds a line to remove the files that are still in
ext and the directories that are still in ext to allow the rmdir to
succeed cleanly.

The failures look like this:

```
rmdir ext		# should be emptied by now
rmdir: failed to remove 'ext': Directory not empty
make[1]: *** [override_dh_auto_install] Error 1
```