R1:ef85f49adc07

Delete license headers from files

Summary:
This commit doesn't change license of any file. It just makes the license implicit (inherited from LICENSE file in the root directory).

We are removing the headers for these reasons:

- It wastes space in editors, less code is visible in editor upon opening a file.
- It brings noise to diff of the first change of any file every year.
- It confuses Git file copy detection when creating small files.
- We don't have an explicit license header in other files (JS…