Change Scala license to unmodified 3-clause BSD.

No longer using the slightly reworded 3-clause BSD license.
This does not change the meaning of the license,
just aligns it with the standard wording.
Merge pull request #2881 from adriaanm/license-cleanup

Change Scala license to unmodified 3-clause BSD.
SI-7767 avoid rejecting Scaladoc comments in early initializers

review by @retronym