Remove ko.utils.setTextContent from public knockout utils API surface.

Although this function is available via knockout-{version}.debug.js, it is
not part of the minified knockout-{version}.js file. Invoking this function
will cause a runtime javascript error when invoked against the minified js.

See this for more info: https://github.com/SteveSanderson/knockout/issues/670
Merge pull request #338 from danludwig/danludwig/work

Remove ko.utils.setTextContent from public knockout utils API surface.