Remove duplicated QDebug operator for GpgME::Key

It is part of debug.h since 0a90ffb7c212fbc2394792265f6e30783639862e
hence in some situations compilation fails with "ambiguous overload for
‘operator<<’ (operand types are ‘QDebug’ and ‘const GpgME::Key’)"

(cherry picked from commit 60bb0ae63eff224f04c2d76f942110b06e9cf570)