AudioCodingModuleImpl: Stop failing artificially for non-Opus encoders

All encoders already handle the "Opus-specific" requests sanely (by
failing nicely), so we don't need extra checks to protect them.

BUG=webrtc:5028

Review URL: https://codereview.webrtc.org/1527453005

Cr-Commit-Position: refs/heads/master@{#11051}
diff --git a/webrtc/modules/audio_coding/test/TestVADDTX.cc b/webrtc/modules/audio_coding/test/TestVADDTX.cc
index a7d2c21..229dc2d 100644
--- a/webrtc/modules/audio_coding/test/TestVADDTX.cc
+++ b/webrtc/modules/audio_coding/test/TestVADDTX.cc
@@ -234,10 +234,10 @@
 // Following is the implementation of TestOpusDtx.
 void TestOpusDtx::Perform() {
 #ifdef WEBRTC_CODEC_ISAC
-  // If we set other codec than Opus, DTX cannot be toggled.
+  // If we set other codec than Opus, DTX cannot be switched on.
   RegisterCodec(kIsacWb);
   EXPECT_EQ(-1, acm_send_->EnableOpusDtx());
-  EXPECT_EQ(-1, acm_send_->DisableOpusDtx());
+  EXPECT_EQ(0, acm_send_->DisableOpusDtx());
 #endif
 
 #ifdef WEBRTC_CODEC_OPUS