Remove tree lines MR-widget

This issue is part of https://gitlab.com/gitlab-org/gitlab-ce/issues/33095

This issue will properly fix https://gitlab.com/gitlab-org/gitlab-ce/issues/32716 so we can simplify:

To see whether it is possible to remove those tree images.

Design/Proposal

This issue cannot go without https://gitlab.com/gitlab-org/gitlab-ce/issues/33976 being implemented!

• Remove the tree lines completely.. they are no longer necessary if https://gitlab.com/gitlab-org/gitlab-ce/issues/33976 is implemented. cc: @fatihacet See mockup:

cc: @victorwu (making the https://gitlab.com/gitlab-org/gitlab-ce/issues/33095 more readable)