Remove SystemJS usage, in development mode, from the worker

Now that https://bugzilla.mozilla.org/show_bug.cgi?id=1247687 has landed in Firefox, we're able to use worker-modules during development :-)

This removes the final piece of SystemJS usage from the PDF.js library, thus allowing a fair bit of clean-up, and we now use *only* native `import`/`export` statements everywhere in development mode.

src/core/unicode.js

@@ -12,7 +12,6 @@
  * See the License for the specific language governing permissions and
  * limitations under the License.
  */
-/* no-babel-preset */
 
 import { getLookupTableFactory } from "./core_utils.js";