C#: Lower the precision of the cs/useless-assignment-to-local to medium.

Author: michaelnebel

csharp/ql/src/Dead Code/DeadStoreOfLocal.ql

@@ -9,7 +9,7 @@
  *       maintainability
  *       useless-code
  *       external/cwe/cwe-563
- * @precision very-high
+ * @precision medium
  */
 
 import csharp