drm/ttm: remove the outdated kerneldoc section

Clean up to start over with new and more accurate documentation.

Signed-off-by: Christian König <christian.koenig@amd.com>
Reviewed-by: Matthew Auld <matthew.auld@intel.com>
Link: https://patchwork.freedesktop.org/patch/msgid/20210908132933.3269-2-christian.koenig@amd.com