Remove unnecessary check_plain() calls from parameters passed to API functions

This module calls check_plain() on search input keys before passing them to API functions like node_search_execute() and user_search_execute().

This seems unnecessary because these are just API functions that are using the data; they are not printing it to HTML.

Also, they were already run through check_plain() before, via this code:

 $result = call_user_func("_spotlight_search_{$category}", check_plain($key));

So a double check_plain() is happening, which is definitely not right.

I think the check_plain() call could probably be removed from there too, but since it's calling an arbitrarily-named function and I don't know for sure all the places that might implement that and how they might use the passed-in $key, I left that off for now and just removed it from the node_search_execute() and user_search_execute() calls.