Cleanup old rake tasks

* accounts:run_deletions: was added with 0.4.0.0 two years ago for one-time usage.
* db:rebuild: db:reset does basically the same
* db:integration:preprare: the integration environments are not used.
* db:reset: there is a default db:reset, no need to write our own.
* db:drop_integration: the integration environments are not used.
* db:fix_diaspora_handle: really old migration from 2010
* db:move_private_key: also old migration from 2010
* maintenance:clear_carrierwave_temp_uploads: tmp/uploads doesn't exist anymore. And we have CleanCachedFiles as cronjob.
* maintenance:install_logrotate_config: diaspora has built-in logrotate support now, and people who want to use logrotate instead can write their own configs with the values they want.
* migrations:copy_hidden_share_visibilities_to_users: old migration from 2012
* migrations:invitations: legacy invitations were removed with #6976
* migrations:absolutify_image_references: old migration from 2010
* migrations:rewire_uppercase_hashtags: old migration from 2012
* migrations:remove_uppercase_hashtags: old migration from 2012