xwayland: Do not discard frame callbacks on allow commits

Currently, when a X11 client (usually the X11 window manager from a
Wayland compositor) changes the value of the X11 property
`_XWAYLAND_ALLOW_COMMITS` from `false` to `true`, all pending frame
callbacks on the window are discarded so that the commit occurs
immediately.

Weston uses that mechanism to prevent the content of the window from
showing before it's ready when mapping the window initially, but
discarding the pending frame callbacks has no effect on the initial
mapping of the X11 window since at that point there cannot be any frame
callback on a surface which hasn't been committed yet anyway.

However, discarding pending frame callbacks can be problematic if we
were to use the same `_XWAYLAND_ALLOW_COMMITS` mechanism to prevent
damages to be posted before the X11 toplevel is updated completely
(including the window decorations from the X11 window manager).

Remove the portion of code discarding the pending frame callback,
Xwayland should always wait for a pending frame callback if there's one
before posting new damages.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Pekka Paalanen <pekka.paalanen@collabora.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
!333
(cherry picked from commit 66da95a1)
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