nextcloud-client: clean up

Continuation of https://github.com/NixOS/nixpkgs/pull/67306

* Switched to non-deprecated dbus config path.
* Removed CMake flags for finding dependencies, the project can find them on its own.
* Properly fixed the RPATH issue from https://github.com/NixOS/nixpkgs/pull/15406 – ${CMAKE_INSTALL_PREFIX}/${CMAKE_INSTALL_LIBDIR}.
* Dropped changing Icon name in the desktop file as it was fixed upstream.
* Switched to fetchFromGitHub, as the submodules are only for Win, Mac & crash reporting.
* Dropped enableParallelBuilding = true, as it is on by default with CMake.