Remove gcc 6 from the list of CI scenarios

It is not supported by Dune 2.8 anymore.
2 jobs for use-if-constexpr in 6 minutes and 34 seconds (queued for 2 seconds)
Status Name Job ID Coverage
  Test
passed dune:git clang C++17 #91768

00:04:39

passed dune:git gcc-8 C++17 #91769

00:06:34