You need to connect with a different user than SYSDBA, and drop the role SYSDBA.

This can be done by:

1. The database owner.

   Find the database owner with:

   select distinct RDB$OWNER_NAME 
   from RDB$RELATIONS 
   where RDB$SYSTEM_FLAG = 1
   

2. The role owner.

   Find the role owner with:

   select RDB$OWNER_NAME
   from RDB$ROLES
   where RDB$ROLE_NAME = 'SYSDBA'
   

3. A user with the role RDB$ADMIN in that specific database if they specify that role when connecting.

   Find members of RDB$ADMIN with:

   select RDB$USER
   from RDB$USER_PRIVILEGES
   where RDB$PRIVILEGE = 'M'
   and RDB$RELATION_NAME = 'RDB$ADMIN'
   

Above queries can be executed by any user, even unprivileged users. If needed you can create a user — with SYSDBA — by connecting to the employee example database, or any other database, and then connect to the database in question.

Find out the owner or a privileged user, create that user (or if it already exists, use or change its password), and then with any of those users, simply execute:

drop role SYSDBA;

(In case of RDB$ADMIN members, make sure to specify the role when connecting.)

If the owner of the database and role is SYSDBA, and there are no users with the RDB$ADMIN role, then you’ll be in a bit of a bind, and I think the only way to fix it then is to use a hex editor, find the right occurrence of SYSDBA, and change it (e.g. to SYSDBB; use the same number of characters!). It is important that you change the right occurrence of SYSDBA, because if you change the wrong occurrence, I think you might break other privileges of SYSDBA.