QBluetoothSocket: remove setOpenMode() calls from ctors

These are no-ops:

The setOpenMode() method is a QIODevice one, and NotOpen is
QIODevice's initial state. So setOpenMode()'s assignment of its arg to
openMode, the assignment of Unset to accessMode, the
setReadChannelCount(0) and the setWriteChannelCount(0) are all no-ops.

Remove them. They only run unnecessary code at runtime and confuse
readers of the code.

Amends fa7615b4e98ec7319e6a264c2a6686949037f6bd (5.4), which replaced
the ReadWrite arguments with NotOpen, turning a wrong call (QTBUG-32704)
into a no-op.

Pick-to: 6.12 6.8
Change-Id: Ie1b37fe5cb0d8fb519a47b0c3145c4a94deb481d
Reviewed-by: Oliver Wolff <oliver.wolff@qt.io>