Drop redundant declaration

Drop redundant declaration

environ is exported in <unistd.h>.

    env.c:29:15: warning: redundant redeclaration of 'environ' [-Wredundant-decls]
       29 | extern char **environ;
          |               ^~~~~~~
    login.c:92:15: warning: redundant redeclaration of ‘environ’ [-Wredundant-decls]
       92 | extern char **environ;
          |               ^~~~~~~
    sulogin.c:40:15: warning: redundant redeclaration of ‘environ’ [-Wredundant-decls]
       40 | extern char **environ;
          |               ^~~~~~~
    newgrp.c:32:15: warning: redundant redeclaration of ‘environ’ [-Wredundant-decls]
       32 | extern char **environ;
          |               ^~~~~~~