`cylc broadcast -k` does nothing

[MetRonnie]

Description

The -k/--display-task option of cylc broadcast does nothing. There are no tests for it either.

Reproducible Example

$ cylc broadcast wflow -p 1 -n foo --set "script=echo 0"
Broadcast set:
+ [1/foo] script=echo 0

then

$ cylc broadcast wflow -k 1/foo

(nothing)

Expected Behaviour

This should work, producing similar output to the -d/--display option.

Alternatively, we could remove this option and instead just implement #6873 which would make it easy to get the same result using e.g. the jq command

[MetRonnie]

Note #6873 has been implemented in #6892 now

We could remove the -k option and make it so --display works in combination with --name and --point