[bugfix] remove additional header install command

This patch removes an extra header install command for tests. It
originally came when copying dune/common/CMakeLists.txt to
dune/common/std/CMakeLists.txt. Sorry for the noise.

dune/common/std/CMakeLists.txt

 install(FILES
         constexpr.hh
 DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/common/std)
-
-# Install some test headers, because they get used by tests in other modules
-# We do this here as test will not be considered for make install
-install(FILES test/iteratortest.hh DESTINATION ${CMAKE_INSTALL_INCLUDEDIR}/dune/common/test)