Just open System Prefs > User & Groups. Click the padlock (bottom left), it usually is locked, enter the master password. Then select your Admin user from the list on the left, click the Password tabs then 'change password' button. Enter the master password in the 'old password' field but leave the 'new password' & 'verify' field empty, the click 'change password'.
Now you will not have a master admin user password.