[bugfix] Clean up ProofreadPage.text
[bugfix] Clean up ProofreadPage.text

BasePage._text doesn't cache the live content anymore,
so don't rely on the internal side effect and just
return the value from the parent.

Bug: T260472
Change-Id: I21e4f234ac75237524d1d8c29c07efe0c45040bf

wikimedia/pywikibot-core,wikimedia/pywikibot-core