coreutils: remove conflicting _FORTIFY_SOURCE definition

[veprbl]

Originally this was introduced in 055e646 ('coreutils: Guard against compiler not supporting __builtin_stpncpy_chk') four years ago. Right now this doesn't seem to serve any purpose and it conflicts with the hardening flags:

<command line>:2:9: warning: '_FORTIFY_SOURCE' macro redefined [-Wmacro-redefined]
#define _FORTIFY_SOURCE 2
        ^
<command line>:1:9: note: previous definition is here
#define _FORTIFY_SOURCE 0
        ^
1 warning generated.

Motivation for this change

Things done

• Tested using sandboxing (nix.useSandbox on NixOS, or option build-use-sandbox in nix.conf on non-NixOS)
• Built on platform(s)
  • macOS
• Tested via one or more NixOS test(s) if existing and applicable for the change (look inside nixos/tests)
• Tested compilation of all pkgs that depend on this change using nix-shell -p nox --run "nox-review wip"
• Tested execution of all binary files (usually in ./result/bin/)
• Fits CONTRIBUTING.md.

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[mention-bot]

@veprbl, thanks for your PR! By analyzing the history of the files in this pull request, we identified @Ericson2314, @DavidEGrayson and @edolstra to be potential reviewers.

[Mic92]

Can you target this pull request against the staging branch (since it is a mass-rebuild)?