ci: Remove ancient and problematic -lstdc++fs in mpexample

This is no longer needed, according to
https://en.cppreference.com/w/cpp/filesystem.html#Notes:

> Using this library may require additional compiler/linker options. GNU
> implementation prior to 9.1 requires linking with -lstdc++fs and LLVM
> implementation prior to LLVM 9.0 requires linking with -lc++fs.

Also, it is causing a OpenBSD link error:

[54/59] Linking CXX executable example/mpexample
FAILED: example/mpexample
: && /usr/bin/c++ -Werror -Wall -Wextra -Wpedantic -Wno-unused-parameter -Xlinker --dependency-file=example/CMakeFiles/mpexample.dir/link.d example/CMakeFiles/mpexample.dir/example.cpp.o example/CMakeFiles/mpexample.dir/init.capnp.c++.o example/CMakeFiles/mpexample.dir/init.capnp.proxy-client.c++.o example/CMakeFiles/mpexample.dir/init.capnp.proxy-server.c++.o example/CMakeFiles/mpexample.dir/init.capnp.proxy-types.c++.o example/CMakeFiles/mpexample.dir/calculator.capnp.c++.o example/CMakeFiles/mpexample.dir/calculator.capnp.proxy-client.c++.o example/CMakeFiles/mpexample.dir/calculator.capnp.proxy-server.c++.o example/CMakeFiles/mpexample.dir/calculator.capnp.proxy-types.c++.o example/CMakeFiles/mpexample.dir/printer.capnp.c++.o example/CMakeFiles/mpexample.dir/printer.capnp.proxy-client.c++.o example/CMakeFiles/mpexample.dir/printer.capnp.proxy-server.c++.o example/CMakeFiles/mpexample.dir/printer.capnp.proxy-types.c++.o -o example/mpexample  libmultiprocess.a  -lstdc++fs  /usr/local/lib/libcapnp-rpc.a  /usr/local/lib/libcapnp.a  /usr/local/lib/libkj-async.a  /usr/local/lib/libkj.a  -lpthread  -Wl,-rpath-link,/usr/X11R6/lib:/usr/local/lib && :
ld: error: unable to find library -lstdc++fs
c++: error: linker command failed with exit code 1 (use -v to see invocation)

Author: marco

example/CMakeLists.txt

@@ -24,6 +24,5 @@ add_executable(mpexample
 target_capnp_sources(mpexample ${CMAKE_CURRENT_SOURCE_DIR} init.capnp calculator.capnp printer.capnp)
 target_include_directories(mpexample PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})
 target_link_libraries(mpexample PRIVATE Threads::Threads)
-target_link_libraries(mpexample PRIVATE stdc++fs)
 
 add_custom_target(mpexamples DEPENDS mpexample mpcalculator mpprinter)