Remove a workaround for old lxml.

scrapy/http/request/form.py

@@ -198,12 +198,6 @@ def _select_value(ele: SelectElement, n: str, v: str):
         # And for select tags without options
         o = ele.value_options
         return (n, o[0]) if o else (None, None)
-    if v is not None and multiple:
-        # This is a workaround to bug in lxml fixed 2.3.1
-        # fix https://github.com/lxml/lxml/commit/57f49eed82068a20da3db8f1b18ae00c1bab8b12#L1L1139
-        selected_options = ele.xpath(".//option[@selected]")
-        values = [(o.get("value") or o.text or "").strip() for o in selected_options]
-        return n, values
     return n, v