QtGlobal: remove qIsDetached()

QtGlobal: remove qIsDetached()

There's not a single in-tree user of this function, and
the concept is a broken one in MT programs: By the time
qIsDetached() returns, the result can already be
different due to another thread taking a copy, or a
copy in another thread being destroyed (note that this
doesn't require mutex use by the user, since we promise
(implicitly, if not explicitly) that you can copy from
const objects without holding a lock).

QTBUG-10813 talks about a use in QCache::trim(), but
677cf76340f88e0fe51c1f75aa512b6d835414ca removed it, so
there's no reason to keep it anymore.

Change-Id: I20380c12bdf00ac764b89d84392f0f34727b1971
Reviewed-by: Lars Knoll <lars.knoll@nokia.com>