[ Merge of http://go/wvgerrit/186825 ]
Remove the words ` All rights reserved.` from Widevine's
existing copyright headers.
For context, see cl/578224540
Bug: 330655176
Test: WVTS
PiperOrigin-RevId: 580020267
Change-Id: I43e845b83f438e4ef7f0f542c2f4e427a188ab06