cmake: remove checking for GCC 5.1

cmake: remove checking for GCC 5.1

this reverts bcc0511d3b857a8810493ca3d3f17ba053651af2,

because

* we already check for C++17 support in the same cmake list
* it's non trivial to check the O(1) std::list::size().
* we always build using the GCC 7.3 from SCLs on CentOS and RHEL. so the
  error message is always printed if we could have detected the O(1)
  std::list::size(). and we cannot fail the build if we are using an
  standard library with O(1) std::list::size().

so, this check is pointless.

Signed-off-by: Kefu Chai <kchai@redhat.com>