Remove client_disconnect().

Remove client_disconnect().

Everything this function does can be done as well when the task
terminates by returning negative in ->post_select().

This also avoids to leak the stream cipher structure of the previous
connection when running in interactive mode.

	1,032 bytes in 1 blocks are definitely lost in loss record 61 of 85
	   at 0x402994A: malloc (vg_replace_malloc.c:263)
	   by 0x804C689: para_malloc (string.c:71)
	   by 0x804FF87: sc_new (crypt.c:268)
	   by 0x804E204: client_post_select (client_common.c:370)
	   by 0x804D203: schedule (sched.c:59)
	   by 0x804A2E7: main (client.c:500)

	1,032 bytes in 1 blocks are definitely lost in loss record 62 of 85
	   at 0x402994A: malloc (vg_replace_malloc.c:263)
	   by 0x804C689: para_malloc (string.c:71)
	   by 0x804FF87: sc_new (crypt.c:268)
	   by 0x804E217: client_post_select (client_common.c:371)
	   by 0x804D203: schedule (sched.c:59)
	   by 0x804A2E7: main (client.c:500)