Remove --sim-only

[leighmcculloch]

I propose we remove the --sim-only option.

The --sim-only option was added recently when we also added --build-only, but it duplicates other functionality we're adding in the stellar tx simulate command.

Removing --sim-only simplifies the varying ways that each command needs to run. It also reduces the varying situations that the different options can conflict in which increases the maintenance burden (e.g. we're having conversations about them on new PRs).