Remove unused HTMLTextFormControlElement::maxLength.

No one calls maxLength for HTMLTextFormControlElement though
HTMLInputElement::maxLength and HTMLTextAreaElement::maxLength are used. We can
remove HTMLTextFormControlElement::maxLength, and stop overriding it in
HTMLInputElement and HTMLTextAreaElement.

BUG=none
TEST=none; no behavior changes.

Review URL: https://codereview.chromium.org/137783010

git-svn-id: svn://svn.chromium.org/blink/trunk@165129 bbb929c8-8fbe-4397-9dbb-9b2b20218538