remove register keyword (in unused code) as it is no longer accpted in C++17 with clang

src/domain/UBGraphicsItemDelegate.cpp

@@ -1307,7 +1307,7 @@ void MediaTimer::setNumDigits(int numDigits)
     } else {
         if (numDigits == ndigits)             // no change
             return;
-        register int i;
+        int i;
         int dif;
         if (numDigits > ndigits) {            // expand
             dif = numDigits - ndigits;