don't bother linking

This, in concert with boxen/puppet-boxen#41, fixes the repeated `icu4c` install/link on every boxen run:

```
Notice: /Stage[main]/Icu4c/Package[icu4c]/ensure: created
Notice: /Stage[main]/Icu4c/Exec[brew link icu4c]/returns: executed successfully
```

In [init.pp#L15-18](https://github.com/boxen/puppet-icu4c/blob/master/manifests/init.pp#L15-L18) we execute `brew link icu4c`. Except we don't really:

```$ brew link icu4c
Warning: icu4c is keg-only and must be linked with --force
Note that doing so can interfere with building software.

$ echo $?
0
```

It appears to succeed but doesn't do anything, because it returns 0 :(

Anyway, that means that alllll this time we've been pretending to link icu4c but not actually doing it, and nobody has complained, so I vote we just stop pretending and make boxen shut up about it.