DefaultOpMatchingService: simplify conversion logic

It used to be the case that ModuleItem could not provide a generic Type,
but rather only a Class. However, that changed back in April:

https://github.com/imagej/imagej/commit/ab4d8638c38df676eeabc7a1a226566d21c6c9af

Since then, we no longer need the special case for CommandModuleItem,
but instead can simply call ModuleItem#getGenericType() always.