cpp: Remove obsolete compatibility with KF5 variants
* lang/cpp/src/GpgmeppConfig-w32.cmake.in.in, lang/cpp/src/GpgmeppConfig.cmake.in.in: Remove find_package() call.
The KF5 variants have been removed years ago with revision
a3cf30f89418c8a6bc9456533d95ba7fc2a33a4c.