Mail: removed dead s->arg_start handling. As discussed in the previous change, s->arg_start handling in the "done" labels of ngx_mail_pop3_parse_command(), ngx_mail_imap_parse_command(), and ngx_mail_smtp_parse_command() is wrong: s->arg_start cannot be set there, as it is handled and cleared on all code paths where the "done" labels are reached. The relevant code is dead and now removed.