Remove HTMLInputElement::isText and isTextType.

They are same and very confusing.
isText is used outside of HTMLInputElement.  It can be replaced with
|isTextField() && type() != InputTypeNames::number|.
isTextType is used only in HTMLInputElement. Its callsites should be
replaced with virtual function dispatches.

BUG=

Review URL: https://codereview.chromium.org/551283002

git-svn-id: svn://svn.chromium.org/blink/trunk@181614 bbb929c8-8fbe-4397-9dbb-9b2b20218538