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/acm2/audio_coding_module_unittest.cc

@@ -1811,7 +1811,6 @@ TEST_F(AcmSenderBitExactnessOldApi, External_Pcmu_20ms) {
   MockAudioEncoder mock_encoder;
   // Set expectations on the mock encoder and also delegate the calls to the
   // real encoder.
-  EXPECT_CALL(mock_encoder, Die());
   EXPECT_CALL(mock_encoder, SampleRateHz())
       .Times(AtLeast(1))
       .WillRepeatedly(Invoke(&encoder, &AudioEncoderPcmU::SampleRateHz));