message.js: trying to set character set throws exception, prevents msg display

Trying to force the character set to UTF-8 here throws a
NS_ERROR_XPC_CANT_MODIFY_PROP_ON_WN exception and prevents
the conversation window displaying at all in Thunderbird 31.0.

Commenting out these 3 lines fixes the problem.

This is more of a "this is a bug with a hacky solution" pull request, since I don't know what the wider ramifications of doing this are - but on my desktop I was having a lot of problems with conversations and particularly new messages never displaying. Activating debug mode pointed me at these 3 lines, and commenting them out has made everything work again.