idl: remove unused functions
idl: remove unused functions

Remove the following functions since they are not used:
* `open_namespaces`
* `close_namespaces`
* `flat_template`

Signed-off-by: Pavel Solodovnikov <2cf5887fafb117b7ff3e29b9720a7305c3313494@scylladb.com>

scylladb/scylla,scylladb/scylla,scylladb/scylla,scylladb/scylla