How do I remove my original Apple ID email which is now described as “an alias”

If it truly is an alias see: Add and manage email aliases for iCloud Mail on iCloud.com - Apple Support

If you were a member of DotMac you got an @mac.com address. If you were a member of MobileMe you got an @me.com address. Now you have an @icloud.com address. So if you go back as far as DotMac you have all three. Confusingly, Apple regards the earlier versions as 'aliases' to the iCloud address (which is a different matter from 'email aliases', which are additional addresses).

These @mac.com and @me.com 'aliases' are the same as the @icloud.com address and cannot be deleted (unlike 'email aliases' which can be deleted as Llmnos has pointed out).

If you are getting unwanted mail to one of these addresses you could set up a Rule online at icloud.com to move any messages to that address to the Trash, from where they will be automatically deleted after one week.