Drop unnecessary assignment

In this branch, !IS_RAW_DATA has already established that offset is
sizeof(QByteArrayData) and realloc maintains the assumption.

Change-Id: Ic160e36d7781d4c4f64a3b2ebec98c9cb605b3eb
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>
Reviewed-by: Jędrzej Nowacki <jedrzej.nowacki@nokia.com>

src/corelib/tools/qbytearray.cpp

@@ -1459,7 +1459,6 @@ void QByteArray::reallocData(uint alloc, bool grow)
         Data *x = static_cast<Data *>(::realloc(d, sizeof(Data) + alloc));
         Q_CHECK_PTR(x);
         x->alloc = alloc;
-        x->offset = sizeof(QByteArrayData);
         d = x;
     }
 }