This was never supported, and the flag is thus confusing. Since the
only supported format is "json" (the default), the flag is redundant
now. However, I've kept it so if someone uses `opa exec --format=json`
somewhere, their calls won't become invalid.
If someone had been using `opa exec --format=pretty`, they will now
see an error, but I think that's less confusing: pretty formatting
is just not supported for 'opa exec'.
Signed-off-by: Stephan Renatus <stephan@styra.com>
Stephan Renatus
ed14340916