Move Remove source branch and Squash commits out of merge request edit web form

• Similar to https://gitlab.com/gitlab-org/gitlab-ce/issues/40680, we want to move components out from the merge request edit web from. In this issue, we want to take these two checkboxes out:
  • Remove source branch when merge request is accepted.
  • Squash commits when merge request is accepted.
• The major consideration here is that these two fields are associated with the merge request, and changed via the edit form, but they can also be overriden right before a user merges the merge request. An example of the different scenarios is here: https://gitlab.com/gitlab-org/gitlab-ce/issues/33264.