I have been suffering from the same problem. The solution I prefer is to disable `auto-save-mode` in mu4e-compose so that drafts do not accumulate. 

 (add-hook 'mu4e-compose-mode-hook #'(lambda () (auto-save-mode -1)))

Plus obviously, set `mu4e-sent-messages-behavior` to `delete`, as suggested by previous answers (especially in Gmail since Gamil takes care of sent messages)

 (setq mu4e-sent-messages-behavior 'delete)