no-op routines in ensemble manager

[hkershaw-brown]

Ensemble manager routines that do nothing. Aim: remove these routines from ensemble_manager and removed any calls to these routines, and references in documentation.

no-op routines (routines that do nothing):
    prepare_to_write_to_vars, prepare_to_write_to_copies, &
    prepare_to_read_from_vars,  prepare_to_read_from_copies, prepare_to_update_vars,  &
    prepare_to_update_copies

Handle has, remove this also:

ens_handle%valid     ! copies modified last, vars modified last, both same

Use build_everything to find which code is calling these routines.