Revert "gstgldisplay: Add public foreign_display property"

Revert "gstgldisplay: Add public foreign_display property"

This introduced a possible regression where the EGL display connection
could be leaked when a foreign native display (x11, wayland, etc) could
create a non-foreign EGL display that would never be destroyed.

The underlying problem needed to be solved in a different way.  See
https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/issues/640
for more details.

This reverts commit 2e686b0dad9700b10d91da5e91f34849fa7d32ae.

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/758>