cleanup: simplify RTCBrowserType.usesPlanB

Define it as !usesUnifiedPlan(). This guarantees that one of the two will be
defined, but only one of them.

modules/RTC/RTCBrowserType.js

@@ -163,11 +163,7 @@ const RTCBrowserType = {
     },
 
     usesPlanB() {
-        return (
-            RTCBrowserType.isChrome()
-                || RTCBrowserType.isOpera()
-                || RTCBrowserType.isReactNative()
-                || RTCBrowserType.isTemasysPluginUsed());
+        return !RTCBrowserType.usesUnifiedPlan();
     },
 
     usesUnifiedPlan() {