Delete redundant asset/symlink removal on Jenkins

The asset pipeline should take care of this for us.

Leaving this code in causes errors of the form:

```
+ export GOVUK_APP_DOMAIN=dev.gov.uk
+ GOVUK_APP_DOMAIN=dev.gov.uk
+ export GOVUK_ASSET_HOST=http://static.dev.gov.uk
+ GOVUK_ASSET_HOST=http://static.dev.gov.uk
+ for d in images javascripts templates stylesheets
+ rm public/images
rm: cannot remove `public/images': No such file or directory
```

Replaces #1024