WIP: overloadsettest: Fix unused capture warning

The warning was caused by a compatibility workaround for GCC5/6. As we don't support those anymore, remove the compatibility workaround rather than adding another workaround on top to suppress the warning.

The warning was cause by run https://gitlab.dune-project.org/core/dune-common/-/jobs/139437#L598 (debian:10 clang-7-libcpp-17):

[ 45%] Building CXX object dune/common/test/CMakeFiles/overloadsettest.dir/overloadsettest.cc.o
/builds/core/dune-common/dune/common/test/overloadsettest.cc:108:10: warning: lambda capture 'dummyCapture' is not used [-Wunused-lambda-capture]
        [dummyCapture](const int& t) { (void) t;},
         ^~~~~~~~~~~~
/builds/core/dune-common/dune/common/test/overloadsettest.cc:109:10: warning: lambda capture 'dummyCapture' is not used [-Wunused-lambda-capture]
        [dummyCapture](const auto& t) { t.bar();});
         ^~~~~~~~~~~~
2 warnings generated.

WIP:

• Wait for CI system to actually drop GCC5/6 testing.