deps: remove @types/extend (#830)

Remove @types/extend as it seems to be broken in combination with Node
12.16.0. Object.assign(...) can be used instead of extend. Only the
deep copy option of extend is not natively supported by Object.assign.

Author: olavloite

test/database.ts

@@ -210,7 +210,9 @@ describe('Database', () => {
         AsyncTransactionRunner: FakeAsyncTransactionRunner,
       },
     }).Database;
-    DatabaseCached = extend({}, Database);
+    // The following commented out line is the one that will trigger the error.
+    // DatabaseCached = extend({}, Database);
+    DatabaseCached = Object.assign({}, Database);
   });
 
   beforeEach(() => {