Au revoir CMake 2.0.0

[tytan652]

Description

Remove CMake legacy code path

Except submodules and legacy_check macro since the former depend on it.

PR for submodules and then the macro removal will be done afterward

Note: Ignoring whitespaces is recommended for reviewing "additions".

Motivation and Context

This path must disapear…

How Has This Been Tested?

Still build on my machine, let's see for CI.

Types of changes

• Code cleanup (non-breaking change which makes code smaller or more readable)

Checklist:

• [x] My code has been run through clang-format.
• [x] I have read the contributing document.
• [x] My code is not on the master branch.
• [x] The code has been tested.
• [x] All commit messages are properly formatted and commits squashed where appropriate.
• [x] I have included updates to all appropriate documentation.