move invocation of (old style) configuration to top-level project

there doesn't appear to be a reason for magically invoking the
configuration from default_pre/post, and it would cause problems further
down the line with the new configure system.

Change-Id: I3cf743a444e96bb397116f2ad4dce923f57aacfc
Reviewed-by: Alexandru Croitor <alexandru.croitor@qt.io>
Reviewed-by: Michal Klocek <michal.klocek@qt.io>
Reviewed-by: Allan Sandfeld Jensen <allan.jensen@qt.io>