xwayland/eglstream: Remove stream validity

To avoid an EGL stream in the wrong state, if the window pixmap changed
before the stream was connected, we would still keep the pending stream
but mark it as invalid. Once the callback is received, the pending would
be simply discarded.

But all of this is actually to avoid a bug in egl-wayland, there should
not be any problem with Xwayland destroying an EGL stream while the
compositor is still using it.

With that bug now fixed in egl-wayland 1.1.7, we can safely drop all
that logic from Xwayland EGLstream backend.

Signed-off-by: Olivier Fourdan <ofourdan@redhat.com>
Reviewed-by: Michel Dänzer <mdaenzer@redhat.com>
Closes: xorg/xserver#1189
(cherry picked from commit 7d509b6f)
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