How to remove dummy account from android?

The answer is within this article:

Add the Account Required by the Framework

The sync adapter framework requires each sync adapter to have an
account type. You declared the account type value in the section Add
the Authenticator Metadata File. Now you have to set up this account
type in the Android system. To set up the account type, add a dummy
account that uses the account type by calling addAccountExplicitly().

So the reason for this account was probably some app, that needed its local database to be synchronized with server using a sync adapter. This app has created account called dummyaccount. Some tricks exist to hide this account from the account list in Settings.