target: remove unused working area 'user' field

working_area::user has never been used so lets remove it.

Change-Id: I1200311b34248549c1fe30c9f675e6129b7bebee
Signed-off-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-on: http://openocd.zylin.com/781


Tested-by: jenkins
Reviewed-by: Freddie Chopin <freddie.chopin@gmail.com>