package/solarus: update license.txt file hash and remove license_gpl.txt hash.

package/solarus: update license.txt file hash and remove license_gpl.txt hash.

The license.txt file has been updated with Solarus Free Resource Pack URL.

Before version 1.6.0, the archive was downloaded from github and license_gpl.txt
file which is present in the git repository.
Since version 1.6.0, the archive is downloaded from [1] but the license_gpl.txt
file is missing.

This has been reported upstream [2].

[1] http://www.solarus-games.org
[2] https://gitlab.com/solarus-games/solarus/issues/1328

Signed-off-by: Romain Naour <romain.naour@gmail.com>
Signed-off-by: Peter Korsgaard <peter@korsgaard.com>