configure: remove AM_MAINTAINER_MODE, effectively always enabling all the rules

There're strong arguments against using this macro (mostly regarding
build consistency), so remove it altogether.

Change-Id: I90c8e9a86a24571019366435bd868a6799a09c45
Signed-off-by: Paul Fertser <fercerpav@gmail.com>
Reviewed-on: http://openocd.zylin.com/1476
Tested-by: jenkins
Reviewed-by: Spencer Oliver <spen@spen-soft.co.uk>
Reviewed-by: Andreas Fritiofson <andreas.fritiofson@gmail.com>