CSSMin: Do not escape U+FFFD as code point

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