Floss: Remove extra clone in cb_variant

For a specific type of callback from libbluetooth, the dispatcher's
reference counter is expected to be increased by 1 each time the callback
is executed and then decreased by 1 before leaving the callback function.
As DispatchContainer::get() has cloned the dispatcher object,
the clone() in cb_variant macro is not necessary.

Bug: 237346241
Tag: #floss
Test: run btadapterd on volteer
Change-Id: I5a175a6f1fc47b943cfb6eb868f4ad650e507b70