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, CSS, images, documentation).
• Using license header in every file is not obligatory: http://www.apache.org/dev/apply-license.html#new.

This change is approved by Alma Chao (Lead Open Source and IP Counsel at Facebook).

Test Plan: Verified that the license survived only in LICENSE file and that it didn't modify externals.

Reviewers: epriestley, davidrecordon

Reviewed By: epriestley

CC: aran, Korvin

Maniphest Tasks: T2035

Differential Revision: https://secure.phabricator.com/D3886