Remove use of OutputPage::addParserOutputNoText()

Simply clicking "Show preview" on the edit page triggered a deprecation
warning.

Also removed the wfDeprecated() call from the method, which is still used
in a few WMF-deployed extensions without a corresponding open change.

Follows-up e8f1fede77c5.

Change-Id: I2cfdc84b92cf13478b9f462028d525e4ec14fdf2