Remove unnecessary locking from QNetworkProxy constructor

Remove unnecessary locking from QNetworkProxy constructor

QGlobalNetworkProxy (a singleton) had two phase construction, with
the second phase being called from QNetworkProxy's constructor.
This isn't necessary, and has been reported as causing deadlocks.

Although constructing socket engine handlers has side effects
(they add themselves to a list on construction and remove themselves
on destruction), this appears to be safe. The socket engine handlers
are only used while holding the list mutex, and any socket engines
created don't have any reference to the factory that created them.

With the new version, it is possible that two instances of
QHttpSocketEngineHandler and QSocks5SocketEngineHandler exist
temporarily if a Q_GLOBAL_STATIC initialisation race occurs.
This appears safe, because the loser of the race deletes its
handlers, which remove themselves from the global list as above.

Task-number: QTBUG-13088
Change-Id: I8cf520da717d8ab7d862ab89c6de13aea6d60ac3
Reviewed-by: Richard J. Moore <rich@kde.org>
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>