Open the PI TimeSeries display from browser.
You can find PI TimeSeries's tag part. (It is table)
Also you can find this table's ID.
After checking id, you can set hidden for this id from css file.
(In my environment, I used C:\Program Files\Common Files\Microsoft Shared\Web Server Extensions\15\TEMPLATE\LAYOUTS\1041\STYLES\Themable\COREV15.CSS)
But if you put multiple PI TimeSeries parts, then the id name is different.
Please check all PI TimeSeries's ID name.
Thank you for your reply. I could fix my issue by your method!