linux-chromiumos: remove 3.14

3.14 is no longer supported upstream by kernel.org and thus no longer
receives security patches. The git commit mentioned in this .nix isn't
even available in the linked repository --
https://chromium.googlesource.com/chromiumos/third_party/kernel -- so I
think this .nix might be dead anyway. Finally, it specifies 3.14.0,
which is so ridiculously old (the latest was 3.14.79) that nobody
develops for it.

Fixes: #25145
Supports: #25127