Manta: Cleanup, strict compiler flag

For some reason was only visible with gcc-10 in release builds.

Kind of makes sense since there is no CMake code which removes strict
compiler flag, so deal with strict flags in the code itself.

intern/mantaflow/intern/MANTA_main.cpp

@@ -2982,6 +2982,7 @@ bool MANTA::updateGridsFromUni(string filename, vector<GridItem> grids)
         return false;
       }
       assert(expectedBytes == readBytes);
+      (void)expectedBytes;
 
       if (with_debug)
         cout << "Fluid: Read successfully: " << filename << endl;