Undeprecations of :class_name I missed last time

[pablobm]

I missed these at #2697. I checked manually if they need to be removed indeed:

• I can confirm that the undeprecation at lib/administrate/search.rb is necessary. I reproduced the scenario described at "The option :class_name is deprecated." but I think it is still needed.  #2384 (comment) and got the warnings when I used the search.
• The other one was a bit trickier as I'm having difficulty putting the scenario together. Still, I did undeprecate it in the docs last time, so it should be undeprecated in the code.

[nickcharlton]

Looks good!