Simplify webinstall plugin message

Mainly wondering why there is a regex for this message when a simple language string would work as well.

I admit the language string isn't very easy to read with all those escape slashes. But that's the only downside I see so far and I think the language teams are capable of handling this.

I also removed the `str_replace` thing which is unneeded as long as the translations are done properly.

If we want to keep the actual URLs out of the language string, we could also use `JText::sprintf()` and pass the URLs as parameters. Personally I don't think we have to expect that those links are changing.

On a sidenote: Can the order of the links and button changed for RTL languages (or maybe also others) with the current approach? I don't think they can but I'm not a pro with regex.