Bug 1162751 - Part 2: Always disable pallete index checking. r=seth

The original assumption was that PR_LOGGING wouldn't be enabled in release
builds. Considering that this warning isn't actionable and PR_LOGGING is now
always defined it's better to just always disable pallete index checking.

image/decoders/nsPNGDecoder.cpp

@@ -286,15 +286,13 @@ nsPNGDecoder::InitInternal()
 #endif
 
 #ifdef PNG_READ_CHECK_FOR_INVALID_INDEX_SUPPORTED
-#ifndef PR_LOGGING
   // Disallow palette-index checking, for speed; we would ignore the warning
-  // anyhow unless we have defined PR_LOGGING.  This feature was added at
-  // libpng version 1.5.10 and is disabled in the embedded libpng but enabled
-  // by default in the system libpng.  This call also disables it in the
-  // system libpng, for decoding speed.  Bug #745202.
+  // anyhow.  This feature was added at libpng version 1.5.10 and is disabled
+  // in the embedded libpng but enabled by default in the system libpng.  This
+  // call also disables it in the system libpng, for decoding speed.
+  // Bug #745202.
   png_set_check_for_invalid_index(mPNG, 0);
 #endif
-#endif
 
 #if defined(PNG_SET_OPTION_SUPPORTED) && defined(PNG_sRGB_PROFILE_CHECKS) && \
             PNG_sRGB_PROFILE_CHECKS >= 0