(Better late than never, maybe it will help the next person.)
Currently, you can't remove a directory. You can, however, add others to it. Edit: you can now delete directories, as long as your currently signed-in user is the last remaining user (and a few other restrictions).
A key point to make before getting into the details is that an Azure Active Directory 'Global Administrator' is only administrator of the directory itself. There is no relation to the Azure subscription, nor does it have an effect on who is admin or co-admin of the subscription.
For others to see your directory when they log in to the Azure portal, you need add them as users of the directory: Azure portal > ACTIVE DIRECTORY > 'Default Directory' > USERS > Add user. You will have the choice of adding their Microsoft Account (MSA) or an Azure Active Directory user.
Tip: You can change the display name of the directory to something more descriptive under the 'CONFIGURE' section.