FIX(ui): Remove mention of variable bitrate from UI

Mumble has been using constant bitrate with Opus for a while now:
https://github.com/mumble-voip/mumble/blob/7a6274f19e2d7c2ffb76b7ebcd22d0c1df9445d9/src/mumble/AudioInput.cpp#L238
All other codecs have been dropped, too.

The UI still mentioned VBR in some places, this commit removes those
mentions.

Fixes #6414

src/mumble/AudioInput.ui

@@ -608,7 +608,7 @@
          <string>Maximum bandwidth used for sending audio</string>
         </property>
         <property name="whatsThis">
-         <string>&lt;b&gt;This shows peak outgoing bandwidth used.&lt;/b&gt;&lt;br /&gt;This shows the peak amount of bandwidth sent out from your machine. Audio bitrate is the maximum bitrate (as we use VBR) for the audio data alone. Position is the bitrate used for positional information. Overhead is our framing and the IP packet headers (IP and UDP is 75% of this overhead).</string>
+         <string>&lt;b&gt;This shows peak outgoing bandwidth used.&lt;/b&gt;&lt;br /&gt;This shows the peak amount of bandwidth sent out from your machine. Audio bitrate is the maximum bitrate for the audio data alone. Position is the bitrate used for positional information. Overhead is our framing and the IP packet headers (IP and UDP is 75% of this overhead).</string>
         </property>
         <property name="text">
          <string/>

src/mumble/AudioStats.ui

@@ -181,7 +181,7 @@
          <string>Bitrate of last frame</string>
         </property>
         <property name="whatsThis">
-         <string>This is the audio bitrate of the last compressed frame (20 ms), and as such will jump up and down as the VBR adjusts the quality. The peak bitrate can be adjusted in the Settings dialog.</string>
+         <string>This is the audio bitrate of the last compressed frame (20 ms). The peak bitrate can be adjusted in the Settings dialog.</string>
         </property>
         <property name="text">
          <string/>