BUG/MINOR: proxy: fix dump_server_state()'s misuse of the trash

BUG/MINOR: proxy: fix dump_server_state()'s misuse of the trash

dump_server_state() claims to dump into a buffer but instead it writes
into a buffer then dumps the trash into the channel, so it only supports
being called with buf=&trash and doesn't need this buffer. There doesn't
seem to be any current impact of this mistake since the function is called
from one location only.

A backport may be performed if it helps fixing other bugs but it will not
fix an existing bug by itself.

(cherry picked from commit 6ff8143f7c87ad40d7369119467d779642702ad5)
Signed-off-by: Christopher Faulet <cfaulet@haproxy.com>