QVoicePrivate: use Rule Of Zero

The copy ctor is exactly the same as the one the compiler would
define, but its presence suggests there might be a difference.

The destructor was already =default'ed, but that might cause
problems on younger C++ standards which seem to require
all-or-nothing semantics.
(cf. e.g. qtbase/fcb57391f73d36914e10ba964dbd9b01fe6f3af2).

Amends 3bd74b6d1cdcefa0a5587ce89af6bda1c98d4b07 (5.8).

Pick-to: 6.11 6.10 6.8
Change-Id: I098e6b18fd04708feb9e01791570c9fcc1ed7ebb
Reviewed-by: Axel Spoerl <axel.spoerl@qt.io>