coreutils: remove conflicting _FORTIFY_SOURCE definition

Originally this was introduced in 055e646b ('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.