Removed a check for a hence unachieavable situation

Now that `Library::getArchive()` never returns a null pointer the check
that was added a few commits ago as a fix for a particular manifestation
of a general problem is no longer needed.

src/readinglistbar.cpp

@@ -39,9 +39,6 @@ void ReadingListBar::setupList()
         } catch (std::out_of_range& e) {
             continue;
         }
-        if ( !archive ) {
-            continue;
-        }
         try {
             auto illustration = archive->getIllustrationItem(48);
             std::string content = illustration.getData();