rhi: gl: Avoid magic adjustments to the context/window format

...by removing the entire adjustedFormat() helper.

Qt Quick has never used this, which indicates it is not that
useful. Same goes for Qt Multimedia or Qt 3D. Ensuring depth and
stencil is requested is already solved by using
QSurfaceFormat::setDefaultFormat() or by adjusting the formats
everywhere as appropriate.

The helper function's usages are in the manual tests that use it as a
shortcut, and in the GL backend itself. Remove it and leave it up the
client to set the depth or stencil buffer size, typically in the
global default surface format. (which in fact many of the mentioned
manual tests already did, so some of calls to
window->setFormat(adjustedFormat()) were completely unnecessary)

By not having the built-in magic that tries to always force depth and
stencil, we avoid problems that arise then the helper cannot be easily
invoked (thinking of widgets and backingstores), and so one ends up
with unexpected stencil (or depth) in the context (where the GL
backend auto-adjusts), but not in the window (which is not under
QRhi's control).

It was in practice possible to trigger EGL_BAD_MATCH failures with the
new rhi-based widget composition on EGL-based systems. For example, if
an application with a QOpenGLWidget did not set both depth and stencil
(but only one, or none), it ended up failing due to the context -
surface EGLConfig mismatches. On other platforms this matters less due
to less strict config/pixelformat management.

Change-Id: I28ae2de163de63ee91bee3ceae08b58e106e1380
Fixes: QTBUG-104951
Reviewed-by: Andy Nichols <andy.nichols@qt.io>
(cherry picked from commit 5b334729d30d89ad7433f7051d94acb21a1938fb)
Reviewed-by: Qt Cherry-pick Bot <cherrypick_bot@qt-project.org>

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