Remove unused Dictionary forward declaration and include

It seems that these are leftovers when I replaced Dictionary
with IDL dictionaries. Added a forward declaration in
Element.h instead (which is actually using Dictionary).

Review URL: https://codereview.chromium.org/723473002

git-svn-id: svn://svn.chromium.org/blink/trunk@185197 bbb929c8-8fbe-4397-9dbb-9b2b20218538