Editorial: Remove "message" property from list of Error differences

[drakedevel]

As noted in https://github.com/tc39/ecma262/issues/1794#issuecomment-558868977 the language about an implementation-defined "message" property in the NativeError section is an outdated holdover from ES3, since the definition was changed to be the empty string in ES5. The outdated language was subsequently duplicated to the Error section in #2040.

Since the message prototype property is defined to be the empty string for all Error types, I thought it would be least confusing to remove it from the "differing only in ..." list entirely.

I am also inclined to update only in the name used as the constructor name instead of _NativeError_ to only in the constructor name in the Error Objects copy, since that clause isn't specific to NativeError. I can roll that into this PR or open a new one if that sounds good.