Remove unneeded fprintf call in CSSPrimitiveValueMappings

The conversions for FontStretch have fprintf calls in them from
https://codereview.chromium.org/203523002/, probably left in by
accident.

Review URL: https://codereview.chromium.org/431853002

git-svn-id: svn://svn.chromium.org/blink/trunk@179282 bbb929c8-8fbe-4397-9dbb-9b2b20218538