Stop using MA_ASSERT in examples.

This is useful because MA_ASSERT is only defined in the implementation
section of miniaudio.h which can cause issues when people copy/paste
the code and use it in a file that does not have visibility of the
implementation.

Note that there are still more references to implementation-defined
macros, but these have been moved to the public section in the dev-0.12
branch so I'm not bothering to change those just yet.

Public issue https://github.com/mackron/miniaudio/issues/787

examples/simple_mixing.c

@@ -87,8 +87,7 @@ void data_callback(ma_device* pDevice, void* pOutput, const void* pInput, ma_uin
     float* pOutputF32 = (float*)pOutput;
     ma_uint32 iDecoder;
 
-    MA_ASSERT(pDevice->playback.format == SAMPLE_FORMAT);   /* <-- Important for this example. */
-
+    /* This example assumes the device was configured to use ma_format_f32. */
     for (iDecoder = 0; iDecoder < g_decoderCount; ++iDecoder) {
         if (!g_pDecodersAtEnd[iDecoder]) {
             ma_uint32 framesRead = read_and_mix_pcm_frames_f32(&g_pDecoders[iDecoder], pOutputF32, frameCount);
@@ -107,6 +106,7 @@ void data_callback(ma_device* pDevice, void* pOutput, const void* pInput, ma_uin
     }
 
     (void)pInput;
+    (void)pDevice;
 }
 
 int main(int argc, char** argv)