Trim off licenses when showing solutions (#1212)

The licenses end up in the PO files, causing extra unnecessary for work
our translators. We save about 300 lines from each PO file with this.

This also solves another small problem: when a file is included with an
anchor, other anchors are automatically stripped away. This removes some
confusing `// ANCHOR: foo` and `// ANCHOR_END: foo` lines in the
solutions.

src/exercises/concurrency/link-checker.rs

@@ -12,6 +12,7 @@
 // See the License for the specific language governing permissions and
 // limitations under the License.
 
+// ANCHOR: solution
 use std::{sync::Arc, sync::Mutex, sync::mpsc, thread};
 
 // ANCHOR: setup