Navigate to your Steam installation folder.
...\steamapps\common\Assassin's Creed Valhalla\videos\

Delete or rename the following videos:

• ANVIL_Logo.webm
  
• PC_AMD_Ryzen.webm
  
• UbisoftLogo.webm

Then go into your language folder folder (for English it's ...\videos\en).

Delete or rename the following videos:

• Epilepsy.webm
  
• PC_WarningSaving.webm
  
• warning_disclaimer.webm

Please be aware that these files might get downloaded again if there's a patch or update to the game. You can easily revert back to the original state by reinstalling the game or using 'Verify Integrity of Game Files'.