4608 - remove media.webspeech.recognition.enable

the 2nd part of the Web Speech API, SpeechRecognition, is not yet implemented on Desktop Firefox:

https://developer.mozilla.org/en-US/docs/Web/API/SpeechRecognition#Browser_compatibility

> * [2] Can be enabled via the `media.webspeech.recognition.enable` flag in about:config **on mobile**. **Not implemented at all on Desktop Firefox** — see [bug 1248897](https://bugzilla.mozilla.org/show_bug.cgi?id=1248897).

user.js

@@ -1628,11 +1628,10 @@ user_pref("browser.zoom.siteSpecific", false);
    // [2] https://wicg.github.io/netinfo/
    // [3] https://bugzilla.mozilla.org/show_bug.cgi?id=960426
 user_pref("dom.netinfo.enabled", false);
-// 4608: disable speech recognition
-   // [1] https://developer.mozilla.org/docs/Web/API/SpeechRecognition
+// 4608: disable the SpeechSynthesis (Text-to-Speech) part of the Web Speech API
+   // [1] https://developer.mozilla.org/docs/Web/API/Web_Speech_API
    // [2] https://developer.mozilla.org/docs/Web/API/SpeechSynthesis
    // [3] https://wiki.mozilla.org/HTML5_Speech_API
-user_pref("media.webspeech.recognition.enable", false);
 user_pref("media.webspeech.synth.enabled", false);
 // 4609: disable location-aware browsing
    // [NOTE] Use Mozilla's API key if required