it's not possible to delete twice-nat or self-twice-nat entries from the nat44 static mappings table using the CLI.

From looking at nat44_cli.c I can see:

      else if (unformat (line_input, "twice-nat"))

        twice_nat = 1;

      else if (unformat (line_input, "del"))

        is_del = 1;

so the delete case can never be hit for twice-nat/self-twice-nat