Revert "Refs #10370 Added reserve back to loadAndAddfrom in MDBox."

This reverts commit 54ad8c6. Decided to leave the reserve as previously, as push back should allow for adequate performance in most cases even if another method relying on loadAndAddFrom in MDBox does not call reserveMemoryForLoad first.

Code/Mantid/Framework/MDEvents/src/MDBox.cpp

@@ -934,9 +934,6 @@ namespace MDEvents
        std::vector<coord_t> TableData;
        FileSaver->loadBlock(TableData,filePosition,nEvents);
 
-       // convert loaded events to data;
-       size_t nCurrentEvents = data.size();
-       this->data.reserve(nCurrentEvents+nEvents);  // if space already reserved, e.g. from MergeMDFiles, this does nothing
        // convert data to events appending new events to existing
        MDE::dataToEvents(TableData,data,false);