contrib/bst-here: Remove redundant create_volume_if_not_exists() method

This method doesn't really do anything as this is already the default
behavior of `docker volume create` so remove it and always call
`docker volume create` directly. This command will always print the name
of the volume on STDOUT which is not very interesting so silence that.
(If it errors out for some reason, that output will go to STDERR so the
user should still be able to see that.)