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 #640 (closed) for more details.
This reverts commit 2e686b0d.