Fix #1946: remove an hard coded title.

The original title is copied if defined and it's sufficient for accessibility purpose.
We shouldn't force the title attribute, it's especially annoying if people are using
tooltip implementation based on the title attribute such as Bootstrap tooltip.

select2.js

@@ -1820,7 +1820,7 @@ the specific language governing permissions and limitations under the Apache Lic
                 "<div class='select2-drop select2-display-none'>",
                 "   <div class='select2-search'>",
                 "       <input type='text' autocomplete='off' autocorrect='off' autocapitalize='off' spellcheck='false' class='select2-input' role='combobox' aria-expanded='true'",
-                "       aria-autocomplete='list' title='Search field' />",
+                "       aria-autocomplete='list' />",
                 "   </div>",
                 "   <ul class='select2-results' role='listbox'>",
                 "   </ul>",