Initially I assigned a specific page view as “Startup Page” using “Set As Startup View” option, but how can I clear this assignment?
I re-clicked “Set As Startup View” option thinking it will clear out the "Startup Page" option, but it didn't work. Also, I though if I set other page as "Startup Page" then it will automatically clear the earlier assignment; however it too didn't work as I could see many pages being getting set as "Startup".

There is one startup view in each layer.

To "clear" the startup view you can set an "empty" view as the startup view for that layer.