When the XBlock runtime is used in Studio/authoring mode, it needs to use the template path prefix "lms." in order to locate the block templates. (In LMS view, no prefix is required.) Fixes bug introduced by https://github.com/openedx/edx-platform/pull/29354 and removes template workaround added by https://github.com/openedx/edx-platform/pull/29517 (see Author Notes & Concerns)