Fix: pathPrefix in ezpLanguageSwitcher

Problem:
When checking wether pathPrefix sould be removed or not, it was assumed that you already were in a subfolder of the PathPrefix. The case where you want to go to the root of your pathPrefix folder wasn't handled.

Ex:
- Home
  - *MyPathPrefixFolder*
    - News
    - Contents
    - ... 
  - MyOtherFolder

Let's say "MyPathprefixFolder" is the home folder for a siteaccess, then you'll have in your site.ini 
```
PathPrefix=MyPathPrefixFolder
```

When switching language directly from your homepage, the PathPrefix was added to the new language:
`mywebsite.com/switchlanguage/to/fr/` redirects to `fr.mywebsite.com/MyPathPrefixFolder` instead of just `fr.mywebsite.com`.

Oh and I also removed unrelevant params.
You could do a 2nd commit, in the same PR, with this version. If people
don't like it, you can just remove the commit. Or you could leave the
substr, it's good too :-)