fix: fix transcript replacement

Fixed an issue where changing the transcript language code would
cause both the old and new transcript to be displayed.
But in this case, you won’t be able to download the transcript
from the old code, since the link is invalid.

xmodule/video_block/video_handlers.py

@@ -467,6 +467,7 @@ class VideoStudioViewHandlers:
 
         return error
 
+    # pylint: disable=too-many-statements
     @XBlock.handler
     def studio_transcript(self, request, dispatch):
         """
@@ -534,6 +535,10 @@ class VideoStudioViewHandlers:
                             'edx_video_id': edx_video_id,
                             'language_code': new_language_code
                         }
+                        # If a new transcript is added, then both new_language_code and
+                        # language_code fields will have the same value.
+                        if language_code != new_language_code:
+                            self.transcripts.pop(language_code, None)
                         self.transcripts[new_language_code] = f'{edx_video_id}-{new_language_code}.srt'
                         response = Response(json.dumps(payload), status=201)
                     except (TranscriptsGenerationException, UnicodeDecodeError):