Remove the MR description template as expected effect on user failed, see #3348

Template will disappear once this is merged to master (until then it will still appear by default in each issue, here I just deleted the suggested text)

.gitlab/merge_request_templates/Default.md

-Please insert x between square brackets to tick the boxes, add a meaningful description if the title is not enough and check the outcome with the "Preview" tab
-
-**In case you committed a new geometry:**
-- [ ] Ran the scripts for materials and radiation length check on my new geometry file? (and read carefully the output of course)
-    ```
-    bash ci_scripts/check_radlen.sh <path>/<file>.geo.root
-    ```
-- [ ] Added also the corresponding creation macro?
-- [ ] Checked that the geometry does not contains unexpected overlaps, e.g. either at end of the creation macro or with the something similar to the mCBM macro \
-`macro/mcbm/mcbm_check_overlaps.C`
-
-**In case you committed a new setup file:**
-- [ ] Checked that all geometries listed are present in the repository?
-- [ ] Ran the CBMROOT `macro/geometry/check_media.C` on the corresponding `.geo.root` file? (output by the transport macro)