git_workflows delete_study takes a commit message, but ignores it

[mtholder]

it should pass it along to the git_action call that does the deletion.

[mtholder]

It looks like we have an uneeded branch of code in git_action.remove_study that deals with the case when only 3 args are passed in. That looks like dead code (we need to check the phylesystem-api repo to verify). After cleaning that up, we can make that function take a commit message.