Always use the Unicode font for non-ASCII

Previously it would still use the ASCII font if the font contained the
character. But this is confusing, and seems to contradict what the option says
it will do. Furthermore, it is more useful this way, as users may prefer the
ASCII characters of one font but the Unicode characters of another font, even
if their ASCII font has those Unicode characters.

This fixes https://code.google.com/p/iterm2/issues/detail?id=1491.

Please note: I am not an Objective-C programmer. This fix seems to work, but you should verify that I didn't do anything stupid.

Also, the function `hasGlyphForCharacter` is not used any more. Should it be removed?