Remove unused header from client.c

plain/plain.h is also server-side implementation of PLAIN
authentication mechanism

Change-Id: I4b65abb79ee0fea118cf976abd67bc429cbb66dc
Reviewed-on: http://review.couchbase.org/29762
Tested-by: Sergey Avseyev <sergey.avseyev@gmail.com>
Reviewed-by: Trond Norbye <trond.norbye@gmail.com>