Removed Die mock from MockAudioEncoder

MockAudioEncoder was calling a mocked Die function on itself in its
destructor. This outputs "Uninteresting mock function call" warning if
the Die call was not expected. This is true even if a NiceMock is used
to suppress the warnings.

The purpose of testing that the destructor is called might be to protect
against memory leaks when audio encoder ownership is transferred using a
raw pointer. However, this case is already covered by msan checks.

Bug: None
Change-Id: I0603c417b4b239027859228e05ebcf83ff5aaf18
Reviewed-on: https://webrtc-review.googlesource.com/56183
Reviewed-by: Karl Wiberg <kwiberg@webrtc.org>
Reviewed-by: Henrik Lundin <henrik.lundin@webrtc.org>
Commit-Queue: Sebastian Jansson <srte@webrtc.org>
Cr-Commit-Position: refs/heads/master@{#22146}

modules/audio_coding/codecs/cng/audio_encoder_cng_unittest.cc

@@ -43,7 +43,6 @@ class AudioEncoderCngTest : public ::testing::Test {
         sample_rate_hz_(8000) {
     memset(audio_, 0, kMaxNumSamples * 2);
     EXPECT_CALL(*mock_encoder_, NumChannels()).WillRepeatedly(Return(1));
-    EXPECT_CALL(*mock_encoder_, Die()).Times(1);
   }
 
   void TearDown() override {

modules/audio_coding/codecs/red/audio_encoder_copy_red_unittest.cc

@@ -49,7 +49,6 @@ class AudioEncoderCopyRedTest : public ::testing::Test {
   }
 
   void TearDown() override {
-    EXPECT_CALL(*mock_encoder_, Die()).Times(1);
     red_.reset();
   }