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

www/otter-browser/pkg-message

@@ -9,8 +9,7 @@ Web=qtwebengine
 
 If you get wrong colors when watching videos while using the QtWebEngine
 backend, this is probably because your system does not support hardware
-acceleration (see bug 237277): you can disable it by starting the browser
-from command line with the --disable-gpu option.
+acceleration (see bug 237277).
 EOM
 }
 ]