[SVG] remove SVGPropertyTrait.h

This is not used in the new SVG property implementation.
All SVG* inherit NewSVGProperty class, and their "traits" are queried through virtual function calls.

TBR=haraken,pdr,fs
BUG=349370

Review URL: https://codereview.chromium.org/190343002

git-svn-id: svn://svn.chromium.org/blink/trunk@168719 bbb929c8-8fbe-4397-9dbb-9b2b20218538