Remove ElementResolveContext::deprecatedPartialClear

I probably could have removed this when I created
ElementResolveContext, but its safer to separate
the behavior change into a separate CL like this.

I don't expect this to actually change observable behavior
all the additional members we are clearing were already
being unconditionally set in initForStyleResolve.

BUG=259202

Review URL: https://chromiumcodereview.appspot.com/19127002

git-svn-id: svn://svn.chromium.org/blink/trunk@154173 bbb929c8-8fbe-4397-9dbb-9b2b20218538