Prevent using image caption in QTI1.2 to not cause problems when migrating to QTI 2.1

In 12.1 we added support for image caption and image title. This is now enabled by default for all rich text formatted elements. Image captions are stored in a figcaption element which is an HTML 5 element. As HTML5 is not supported in QTI 2.1 the image caption is disabled in the QTI 2.1 editor. However, in QTI 1.2 it is enabled, which causes problems when migrating those questions to 2.1. To solve this we disable the image caption also in QTI 1.2