Remove legacy_check CMake macro

[tytan652]

Description

The macro was not removed in 72428cc waiting for submodules to be be updated to a commit where their legacy code path is removed.

Motivation and Context

The macro is non-functional (a stub) but was kept until submodules are updated to no longer require it.

How Has This Been Tested?

No issue configuring and building on Linux, CI will tell for other platforms

Types of changes

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

Checklist:

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

[RytoEX]

Looks fine at a glance.