Remove some unused enum values.

- WEBP_MUX_INVALID_PARAMETER: was used only at one place, and that too
should actually be an assert().
- WEBP_MUX_ERROR: was never used.

Change-Id: I8883cb4dfae7a7918507501f21fced0c04dda36a

src/webp/mux.h

@@ -56,13 +56,11 @@ extern "C" {
 // Error codes
 typedef enum {
   WEBP_MUX_OK                 =  1,
-  WEBP_MUX_ERROR              =  0,
-  WEBP_MUX_NOT_FOUND          = -1,
-  WEBP_MUX_INVALID_ARGUMENT   = -2,
-  WEBP_MUX_INVALID_PARAMETER  = -3,
-  WEBP_MUX_BAD_DATA           = -4,
-  WEBP_MUX_MEMORY_ERROR       = -5,
-  WEBP_MUX_NOT_ENOUGH_DATA    = -6
+  WEBP_MUX_NOT_FOUND          =  0,
+  WEBP_MUX_INVALID_ARGUMENT   = -1,
+  WEBP_MUX_BAD_DATA           = -2,
+  WEBP_MUX_MEMORY_ERROR       = -3,
+  WEBP_MUX_NOT_ENOUGH_DATA    = -4
 } WebPMuxError;
 
 // Flag values for different features used in VP8X chunk.