Remove duplicate license file

[stweil]

Update also the GPL 3 to the latest license text.

[stweil]

I'm afraid this pull request only addresses a small part of the current license issues.

• There is still the problem that it is no clear statement which license applies to Kitodo as a whole (see issue License #1301 which was closed to early in my opinion). 233 files in the repository have no explicit license statement. I don't think that it would be good to add such a statement to each of these files. A better solution would be to explain the license(s) in the README.
• GitHub is confused if there is more than a single license file in the repository root unless there is a file named LICENSE, LICENSE.txt, LICENSE.md or LICENSE.rst. My pull request restores this confusion which is bad.

We could create a license directory again, move all license files to this directory and link license/GPL3-License.txt to LICENSE.txt. But what was the reason for the currently used license headers (introduced in pull request #522)? 1518 files refer to GPL3-License.txt and don't use the recommended header for GPL-3 files.

My favourite solution:

1. Use the commonly used header text which is recommended for GPL-3 files. Then there is no longer a reference to a specific file and GPL3-License.txt can be removed.
2. Add LICENSE.md for GPL-3 (available from gnu.org).
3. Clarify all relevant licenses in README.md.

Would this be okay for everybody? If yes, I could start with 1 and 2 (see my master branch and commit ab04dbc). As far as I know 3 still needs a statement from the Kitodo Board (Vorstand).