Remove ASF license headers on 3rdparty files based on discussion:

> It depends on what you mean by significant, usually changes made to 3rd party files stay under the original license. See [1] on this, note that "Do not add the standard Apache License header to the top of third-party source files.”. If the license was to change, and the headers replaced then that needs to be noted in NOTICE. Please make sure that the license is noted in LICENSE.

https://lists.apache.org/thread.html/r4010c53f976819c94f3450469db8b936f7c15cb046a61f0d488674d3%40%3Cgeneral.incubator.apache.org%3E