client: Remove i386 GNU assembler version of S_WriteLinearBlastStereo16()

This was the last remaining GNU assembler code that was in its own
independent source file, as opposed to being embedded in C code with
GNU `__asm__ volatile` or MSVC `__asm` (which tends to be much easier
to deal with in a modern build system).

When the Quake 3 engine was originally written, this might have been
an "expensive" function when written in portable C, but after 25 years
of CPU development it's less likely to matter (and modern compilers
might produce faster results from the C code anyway).

Resolves: https://github.com/ioquake/ioq3/issues/778
Signed-off-by: Simon McVittie <smcv@debian.org>

code/client/snd_mix.c

@@ -31,7 +31,7 @@ int*     snd_p;
 int      snd_linear_count;
 short*   snd_out;
 
-#if	!id386                                        // if configured not to use asm
+#if defined(__GNUC__) || !id386
 
 void S_WriteLinearBlastStereo16 (void)
 {
@@ -57,10 +57,8 @@ void S_WriteLinearBlastStereo16 (void)
 			snd_out[i+1] = val;
 	}
 }
-#elif defined(__GNUC__)
-// uses snd_mixa.s
-void S_WriteLinearBlastStereo16 (void);
-#else
+
+#else   // MSVC on i386
 
 __declspec( naked ) void S_WriteLinearBlastStereo16 (void)
 {