Enable "Remove branch" by default in MR widget, and remove option from New MR form

remove the 'Remove branch' option when you create the MR
check it by default when you merge the MR, unless there are other MR's using the same source branch, or with that branch as a target branch (or hide it if the source branch is protected, as we already do)

Original description

We should make GitLab safe by default, but @ChadMalchow mentioned that removing the branch is a best practise and should be enabled by default.

Maybe we can do this if we have a way to restore the branch easily if you mistakenly delete it?