Use automatic parallelism options in local.conf.

Since all targets now use current poky version, we can get rid of the
workaround for dora which needed hardcoded values.

Change-Id: Ia720c1da3f75f66a78e3ea09bb86c1182b72fb6d
Reviewed-by: Samuli Piippo <samuli.piippo@theqtcompany.com>