optionmap: remove superfluous code

This was noticed while looking at a false-positive in the report for
PVS-Studio, in #192.

* spot/misc/optionmap.cc: Do not remove the option from unused_ twice..

spot/misc/optionmap.cc

@@ -182,7 +182,6 @@ namespace spot
   int
   option_map::operator[](const char* option) const
   {
-    unused_.erase(option);
     return get(option);
   }