配列の内容を消すときにはdelete

実際のところ

BEGIN {
  PROCINFO["sorted_in"] = "@ind_str_asc"
  ary[3] = "j"
  ary[1] = "t"
  ary[2] = "a"
  ary[0] = "b"
  delete ary[2]

  for (i in ary) 
    print i, ary[i]
}

#>> 0 b
#>> 1 t
#>> 3 j

参考もと

• effective awk