Simplify tokens_owner cleanup in read_from_stream per review

Drop the justification comment and the manual reset/nullptr pair;
letting the unique_ptr go out of scope at function exit already frees
the lexer on every return path.

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src/ifcparse/IfcParse.cpp

@@ -1584,7 +1584,8 @@ void IfcParse::impl::in_memory_file_storage::read_from_stream(IfcParse::FileRead
         return;
     }
 
-    tokens = new IfcSpfLexer(s, logger());
+    std::unique_ptr<IfcSpfLexer> tokens_owner(new IfcSpfLexer(s, logger()));
+    tokens = tokens_owner.get();
 
     std::vector<std::string> schemas;
 
@@ -1689,8 +1690,6 @@ void IfcParse::impl::in_memory_file_storage::read_from_stream(IfcParse::FileRead
 
     logger().Status("\rDone scanning file   ");
 
-    delete tokens;
-
     if (good_ != file_open_status::SUCCESS) {
         return;
     }