unset i at the end of gvm-init.sh to avoid error in zsh

Added "unset i" near the end of the file to not expose the variable to the original shell. When using zsh, that had caused strange behaviour when (re-)using a variable called "i":

```Shell
$ for i in *.pdf; do echo $i ; done
zsh: bad floating point constant
```

See: https://github.com/gvmtool/gvm/issues/231