platforms.nix: Remove now unused kernelMajor
platforms.nix: Remove now unused kernelMajor

The last use of `kernelMajor` in Nixpkgs was removed in 2018.

Even then, I'm not positive it was actually in an exercised code path.

AFAIUI this is now totally redundant and useless as it really was meant
for the 2.4 -> 2.6 transition.

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