LICENSES: Remove CC-BY-SA-4.0 license text

Using non-GPL licenses for our documentation is rather problematic,
as it can directly include other files, which generally are GPLv2
licensed and thus not compatible.

Remove this license now that the only user (idr.rst) is gone to avoid
people semi-accidentally using it again.

Signed-off-by: Christoph Hellwig <[email protected]>
Signed-off-by: Greg Kroah-Hartman <[email protected]>