Remove swiftmailer and mailsystem from Rocketship Core

A symfony_mailer dependency will be a part of dropsolid_rocketship_profile because we need to config split it, no dependency for rocketship_core.

The dependencies of swiftmailer and mailsystem will be removed from composer.json in 6.1.x. So users developers can uninstall the modules before upgrading the rocketship_core to the next minor version.

Available in the new release