Drop LICENSE.txt

The warranty disclaimer probably bears restating, so move it into the
license section of the README.  The rest of the file duplicates
information elsewhere in the repo.

Author: bgilbert

LICENSE.txt

@@ -64,3 +64,8 @@ components so OpenSlide Python can find OpenSlide, see the package
 
 OpenSlide Python is released under the terms of the [GNU Lesser General
 Public License, version 2.1](https://openslide.org/license/).
+
+OpenSlide Python is distributed in the hope that it will be useful, but
+WITHOUT ANY WARRANTY; without even the implied warranty of MERCHANTABILITY
+or FITNESS FOR A PARTICULAR PURPOSE.  See the GNU Lesser General Public
+License for more details.