removed duplicate toc entry

Author: Trevor Bye

articles/cognitive-services/Speech-Service/toc.yml

@@ -129,9 +129,6 @@
       items:
         - name: Improve synthesis with SSML
           href: speech-synthesis-markup.md
-        - name: Improve synthesis with Custom Voice
-          displayName: custom voice
-          href: how-to-custom-voice.md
         - name: Synthesize speech with Long Audio API
           href: long-audio-api.md
         - name: Get a list of voices with REST (Python)