[doc] remove version check for handbook build

We require Dune >= 2.7.

doc/handbook/CMakeLists.txt

-if(NOT (${DUNE_COMMON_VERSION} VERSION_LESS "2.7"))
 # use latexmk
 dune_add_latex_document(
   SOURCE 0_dumux-handbook.tex
   FATHER_TARGET doc
   INSTALL ${CMAKE_INSTALL_DOCDIR}/doc/handbook)
-
-else()
-# use UseLATEX.cmake
-set(TEX_INPUTS
-  0_dumux-handbook.tex
-  0_listingstyle.tex
-  1_introduction.tex
-  2_quickinstall.tex
-  3_detailedinstall.tex
-  4_course.tex
-  4_examples.tex
-  4_furtherpractice.tex
-  5_assemblinglinearsystem.tex
-  5_developingdumux.tex
-  5_externaltools.tex
-  5_newfoldersetup.tex
-  5_parameterfiles.tex
-  5_restartsimulations.tex
-  5_structure.tex
-  6_basics.tex
-  6_inputoutput.tex
-  6_parallel.tex
-  6_propertysystem.tex
-  6_spatialdiscretizations.tex
-  6_stepsofasimulation.tex
-  6_temporaldiscretizations.tex
-  installDumux.sh)
-
-set(TEX_IMAGES
-  png/box_disc.png
-  png/cctpfa.png
-  png/dumux_strucutre_flowchart_horizontal_explained.png
-  png/dunedesign.png
-  ../logo/dumux_logo_hires_whitebg.png
-  png/dalton1.png
-  png/dalton2.png
-  pdf/staggered_grid.pdf
-  pdf/mpfa_iv.pdf)
-
-dune_add_latex_document(0_dumux-handbook.tex
-  BIBFILES dumux-handbook.bib
-  INPUTS ${TEX_INPUTS}
-  IMAGES ${TEX_IMAGES})
-
-create_doc_install(${CMAKE_CURRENT_BINARY_DIR}/0_dumux-handbook.pdf
-  ${CMAKE_INSTALL_DOCDIR} 0_dumux-handbook)
-endif()