Remove a not very important message from the cli

The cli currently does:

        click.echo(
             "Removed '{}', therefore you can no longer run BuildStream "
             "commands from the current directory.".format(element_name),
             err=True,
         ) 

At the end of bst workspace close in the case that the command was issued while CWD was in a workspace that was consequently closed.

This is not even entirely true (it doesn't check CWD or whether bst -C was used), and is not very helpful (the user will find out anyway the next time they try to type a bst command in the removed workspace directory), it does on the other hand come with some extra, redundant and confusing core API which we can thankfully remove.