Remove HTMLBaseFontElement IDL and implementation

This is dead code because it doesn't appear in the GYP files and
HTMLTagNames.in already says "basefont interfaceName=HTMLElement".

BUG=315480

Review URL: https://codereview.chromium.org/61673002

git-svn-id: svn://svn.chromium.org/blink/trunk@161449 bbb929c8-8fbe-4397-9dbb-9b2b20218538