QFlatMap: use erase() in remove()

De-duplicates code.

Pick-to: 6.3 6.2
Change-Id: I95d3d6f57c2f3716b8f3f549d1cc2a02cde9e996
Reviewed-by: Edward Welbourne <edward.welbourne@qt.io>
Reviewed-by: Fabian Kosmale <fabian.kosmale@qt.io>
Reviewed-by: Jörg Bornemann <joerg.bornemann@qt.io>

src/corelib/tools/qflatmap_p.h

@@ -609,8 +609,7 @@ public:
     {
         auto it = binary_find(key);
         if (it != end()) {
-            c.keys.erase(toKeysIterator(it));
-            c.values.erase(toValuesIterator(it));
+            erase(it);
             return true;
         }
         return false;