[pipe] Disallow 24 bit input, doesn't work currently

If 24 bit input needs to be transcoded, like it will for AirPlay, it
will fail in transcode.c, because XCODE_PCM24 actually produces 4-byte
samples.

src/inputs/pipe.c

@@ -1054,7 +1054,7 @@ init(void)
     }
 
   pipe_bits_per_sample = cfg_getint(cfg_getsec(cfg, "library"), "pipe_bits_per_sample");
-  if (pipe_bits_per_sample != 16 && pipe_bits_per_sample != 24 && pipe_bits_per_sample != 32)
+  if (pipe_bits_per_sample != 16 && pipe_bits_per_sample != 32)
     {
       DPRINTF(E_FATAL, L_PLAYER, "The configuration of pipe_bits_per_sample is invalid: %d\n", pipe_bits_per_sample);
       return -1;