audio: Remove stub header SDL_audio_c.h.

It was a leftover, that just included SDL_sysaudio.h, in modern times.

VisualC-WinRT/SDL-UWP.vcxproj

@@ -93,7 +93,6 @@
     <ClInclude Include="..\src\audio\disk\SDL_diskaudio.h" />
     <ClInclude Include="..\src\audio\dummy\SDL_dummyaudio.h" />
     <ClInclude Include="..\src\audio\SDL_audiodev_c.h" />
-    <ClInclude Include="..\src\audio\SDL_audio_c.h" />
     <ClInclude Include="..\src\audio\SDL_sysaudio.h" />
     <ClInclude Include="..\src\audio\SDL_audioqueue.h" />
     <ClInclude Include="..\src\audio\SDL_audioresample.h" />

VisualC-WinRT/SDL-UWP.vcxproj.filters

@@ -177,9 +177,6 @@
     <ClInclude Include="..\src\audio\SDL_audiodev_c.h">
       <Filter>Source Files</Filter>
     </ClInclude>
-    <ClInclude Include="..\src\audio\SDL_audio_c.h">
-      <Filter>Source Files</Filter>
-    </ClInclude>
     <ClInclude Include="..\src\audio\SDL_sysaudio.h">
       <Filter>Source Files</Filter>
     </ClInclude>