QFlatMap: fix pointless reallocations on repeated range-insert()s

When looping over range-insert(), the repeated shrink_to_fit() calls
would cause cause reserved (or geometrically-grown) capacity to be
shed, breaking the underlying container's growth strategy.

Fix by not shedding excess capacity.

Pick-to: 6.3 6.2
Change-Id: I10915a06fc9442039c192486a55e48083da7c839
Reviewed-by: Joerg Bornemann <joerg.bornemann@qt.io>

src/corelib/tools/qflatmap_p.h

@@ -971,8 +971,6 @@ private:
                 k = i + 1;
             }
         }
-        c.keys.shrink_to_fit();
-        c.values.shrink_to_fit();
     }
 
     containers c;