cpp: Fix Key::canSign()

* lang/cpp/src/key.h (canReallySign): Deprecate.
* lang/cpp/src/key.cpp (canSign): Remove workaround. Use implementation
of canReallySign.
(canReallySign): Use canSign().
(operator<<): Use canSign().

The workaround in canSign was added 19 years ago and canReallySign, the
workaround for the workaround, was added 13 years ago. Time to get rid
of those workarounds for a bug in gpgme which has been fixed long ago
and which cause bugs for any unsuspecting user of Key::canSign().

• GnuPG-bug-id: T6456