V2 post-processing step leaves "fragments" directories

This is of course very low priority, as it does not cause any issues AFAIK, but when untar-ing the resulting package from the GAPIC post-processor, I can see that the `fragment` directories are there even though they contain no files (and we don't want them there):

```
$ bazel build google/cloud/support/v2:google-cloud-support-v2-php
$ tar -xvf bazel-bin/google/cloud/support/v2/google-cloud-support-v2-php.tar.gz
google-cloud-support-v2-php/fragments/
google-cloud-support-v2-php/fragments/Google/
google-cloud-support-v2-php/fragments/Google/Cloud/
google-cloud-support-v2-php/fragments/Google/Cloud/Support/
google-cloud-support-v2-php/fragments/Google/Cloud/Support/V2/
google-cloud-support-v2-php/proto/
...
```
We should remove the empty directories in the post processing step to reduce the noise