CSSMin: Do not escape U+FFFD as code point
The current editors draft from 23 April 2018 does not require to escape
the REPLACEMENT CHARACTER (U+FFFD) as code point anymore.
https://drafts.csswg.org/cssom/#serialize-a-string
If the character is NULL (U+0000), then the REPLACEMENT CHARACTER
(U+FFFD).
https://www.w3.org/TR/2016/WD-cssom-1-
20160317/#serialize-a-string
If the character is NULL (U+0000), then the REPLACEMENT CHARACTER
(U+FFFD) escaped as code point.
Change-Id: Ia67e89b3c9561ca29e133d61a2eca8f3db306d8c