Support for the --set-upstream option was removed in 52668846ea
(builtin/branch: stop supporting the "--set-upstream" option,
2017-08-17), after a long deprecation period.
Remove the option from the command synopsis for consistency. Replace
another reference to it in the description of `--delete` with
`--set-upstream-to`.
Signed-off-by: Todd Zullinger <tmz@pobox.com> Signed-off-by: Junio C Hamano <gitster@pobox.com>