Remove mention of a command-line-flag that doesn't work.

There is a discussion of these flags in qutebrowser's bugtracker
(not otter!),
https://github.com/qutebrowser/qutebrowser/issues/2671
but the mechanism of "--qt-flag" is specific to qutebrowser:
it isn't a general Qt thing. I can't find anywhere in otter-browser's
source history where it has a --disable-gpu flag: that is a Qt
thing, as documented at
https://doc.qt.io/qt-5/qtwebengine-debugging.html
However the application has to be able to pass that on to Qt
(which otter apparently doesn't anymore, and neither does falkon).

Overall the recommendation is to use environment variables to
manage Chromium-inside-WebEngine, as documented by Qt,
e.g.
QTWEBENGINE_CHROMIUM_FLAGS="--disable-gpu" otter-browser

PR: 237277 240097