[Fix #11841] Revert "Fix calculation of atlas map extent when set to fixed scale mode"

This reverts commit b106f8225324237175aac741ede9eb5d3268edcd and fixes the setting of the atlas feature in composer.

According to Nyall original commit b106f8225324237175aac741ede9eb5d3268edcd was introduced to fix a test.
He will look to fix the test, but in the mean time we rather need working features rather than passing tests.
One might wants to disable the test?