icu4c: change keg_only reason

Explain why icu4c still keg_only.

`:provided_by_osx` provides the following text automatically:

```
Mac OS X already provides this software and installing another version in
parallel can cause all kinds of trouble.
```

so I think we can drop the secondary text here, or at least shorten it to "OS X provides libicucore.dylib" (does it provide any other conflicting files?)