Require recent GCC and MSCV and removed `EIGEN_HAS_CXX14` and some other feature test macros

This MR removes the EIGEN_HAS_VARIADIC_TEMPLATES and EIGEN_HAS_STATIC_ARRAY_TEMPLATE, EIGEN_HAS_ALIGNAS (which seemed to be unused) as well as EIGEN_HAS_CXX14 and the corresponding version checks.

It also removed the checks for GCC older than 5.1 and MSCV older than 1900.

What is the new minimum version for ICC? I think 1600 would make all the current checks pass. Also, there are some checks that checked EIGEN_MAX_CPP for 14 and then used standard feature-test macros directly. So far I've only removed the MAX_CPP part of the check.

I've updated the list in #2372 (closed)