qremoteobjectsource.cpp: be more explicit on what we delete

In the ranged-for loops over m_children, each 'it' variable is
a QPointer. Calling delete on it is perfectly fine, because it goes
via operator T*().
However, the follow-up change will take 'it' by const-ref instead of
by-value, and then the code will be a bit confusing (i.e. calling
delete on a const ref).

To make it more readable, explicitly call data(), to signal that
we're deleting the underlying data.

Amends d9ee9fff1b9291395ba7ab75d7e46ddf4a9ff3a4.

Pick-to: 6.12 6.11 6.8
Change-Id: I2a9f7d8eff55a7101d7b067a6454e3517b4cc013
Reviewed-by: Tatiana Borisova <tatiana.borisova@qt.io>