[extractors] Use new framework for existing embeds (#4307)

`Brightcove` is difficult to migrate because it's subclasses may depend
on the signature of the current functions. So it is left as-is for now

Note: Tests have not been migrated

yt_dlp/extractor/pladform.py

@@ -1,5 +1,3 @@
-import re
-
 from .common import InfoExtractor
 from ..utils import (
     determine_ext,
@@ -24,6 +22,7 @@ class PladformIE(InfoExtractor):
                         )
                         (?P<id>\d+)
                     '''
+    _EMBED_REGEX = [r'<iframe[^>]+src=(["\'])(?P<url>(?:https?:)?//out\.pladform\.ru/player\?.+?)\1']
     _TESTS = [{
         'url': 'http://out.pladform.ru/player?pl=18079&type=html5&videoid=100231282',
         'info_dict': {
@@ -61,13 +60,6 @@ class PladformIE(InfoExtractor):
         'only_matching': True,
     }]
 
-    @staticmethod
-    def _extract_url(webpage):
-        mobj = re.search(
-            r'<iframe[^>]+src=(["\'])(?P<url>(?:https?:)?//out\.pladform\.ru/player\?.+?)\1', webpage)
-        if mobj:
-            return mobj.group('url')
-
     def _real_extract(self, url):
         video_id = self._match_id(url)