Cleanup [RuntimeEnabled] use in CanvasRenderingContext2D

This removes some cruft added in:
Add versions of stroke, fill, and clip that take a Path parameter.
https://codereview.chromium.org/137353004/
...which was done b/c [RuntimeEnabled] didn't work properly on
overloaded methods.

This has been fixed (thanks Jens!) and thus the cruft can be removed.

R=haraken
BUG=339000

Review URL: https://codereview.chromium.org/334803002

git-svn-id: svn://svn.chromium.org/blink/trunk@176086 bbb929c8-8fbe-4397-9dbb-9b2b20218538