Removed invalid values for 'letter-spacing' and 'word-spacing'

[According to the current draft](http://dev.w3.org/csswg/css-text/#propdef-word-spacing) `letter-spacing` and `word-spacing` do *not* allow multiple values like `1em .5em`, `1em .5em 2em`, or `normal 1em 2em`. Furthermore `letter-spacing` does *not* allow percentages.

So this pull request removes these values.