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 fixes in egl-wayland, we can now safely drop all that logic from Xwayland EGLstream backend.