Don't attempt to delete remote branch after a PR has landed. (#42)

GitHub deletes the remote branch automatically when the PR is merged, so
we don't to explicitly do that. Trying to delete it currently results in
an error like this:
```
error: unable to delete 'ZolotukhinM/stack/1': remote ref does not exist
error: failed to push some refs to '...'
```
The code for deleting the branch has been sitting there for a long time,
but until a fix in #27 it used incorrect branch names.

src/stack_pr/cli.py

@@ -1117,9 +1117,6 @@ def command_land(args: CommonArgs):
     )
 
     delete_local_branches(st, args.verbose)
-    delete_remote_branches(
-        st[:1], args.remote, args.verbose, args.branch_name_template
-    )
 
     # If local branch {target} exists, rebase it on the remote/target
     if branch_exists(args.target):