[cleanup] Stop using deprecated header keywords.hh

Carsten Gräserrequested to merge
cleanup/stop-including-keywords-hh into master
  • Replace macro DUNE_INLINE_VARIABLE by inline, because for the supported compilers it's always defined to this way.
  • Stop including deprecated, no longer used header keywords.hh.
  • Remove conditionally defined anonymous namespace, because for the supported compilers the condition always evaluates to false.
  • Fix indentation after removal of namespace.

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