QBinaryJsonValue: remove dead detach()

QBinaryJsonValue::detach() has had no callers since it was introduced
in qtbase/35adb74ddd915831789f0175423660f8e898942e (5.15).

Mutating a QBinaryJsonArray/QBinaryJsonObject always goes through the
container's own detach(), which operates on the container's `a`/`o`
root pointer, not through this value-level one.

Coverity complained about a use-after-free in this function, which
appears to be a Falso Positive, and Claude, when reviewing a local
change, pointed out that it doesn't check the result of clone(), which
can return nullptr.

Removing the function, as something that isn't present improves
security.

Pick-to: 6.12 6.11 6.8
Coverity-Id: 390858
Change-Id: I643f91667e7967dc424640605904c40c635115ae
Reviewed-by: Thiago Macieira <thiago.macieira@intel.com>