Remove obsolete "arc land" flags: --update-with-merge/rebase, --delete-remote

Summary:
Fixes T12815. During the last update to "arc land", some flags were disabled but remained in place in case we needed to retain them.

It now seems reasonably clear that we do not. The "rebase" and "merge" strategies for landing were replaced by a better "headless" strategy which seems to avoid the original issues, so these flags no longer do anything or reasonably could do anything.

--delete-remote is slightly more ambiguous (e.g., see T12650 and maybe others) but the only real use case is "git push = save changes".

Test Plan:
Ran arc land --update-with-rebase, was told the flag does not exist.
Grepped for affected flags/symbols.

Reviewers: chad

Reviewed By: chad

Maniphest Tasks: T12815

Differential Revision: https://secure.phabricator.com/D18108