Remove obsolete function IDocument::isTextDocument()

This function was introduced in f7990424, then on the same day the same developer made it unnecessary and removed all its uses in 544ababf. Later other developers started calling this function in place of IDocument::textDocument(), possibly assuming it is a more efficient equivalent. isTextDocument() became equivalent to textDocument() in 9277898e in order to work around crashes in the code that used isTextDocument() by mistake.

Now isTextDocument() is never used the way it was originally meant to be. Remove this function and use textDocument() instead.