Do not append -signed to the server zip name

With the current setup, the signing script is returning a series of zips with `-signed` appended to the name and corresponding `md5` files. When including these zips in the downloads pages, the current protocol is:
- move the unsigned zips/md5: `mkdir unsigned && mv OMERO.server* unsigned`
- extract the signed `targ.gz` with all servers 
- rename `OMERO.server*-signed.zip`  as `OMERO.server*.zip`
- regenerate the MD5  with the correct file names without `-signed`
- regenerate the downloads page with the correct md5

The manual step of generating md5, renaming files is at odds with the md5 auto-generation and is likely to induce manual errors. With, this PR, the above could be replaced by simply:

```
mkdir unsigned && mv OMERO.server* && unzip OMERO.server-signed.tar.gz`
```

before regenerating the downloads page.

/cc @manics @kennethgillen