gl/display: remove choosing egl fallback from GST_GL_PLATFORM
If GST_GL_WINDOW is unset but GST_GL_PLATFORM=egl, then we were choosing to create an GstGLDisplayEGL directly instead of going through the any more specific windowing system implementation (X11, Wayland).
The 'create an GstGLDisplayEGL when GST_GL_PLATFORM=egl' was a fallback as we did not have entries for all EGL-using window systems previously. Now that we do, the fallback can be removed. An EGLDisplay can still be created by setting GST_GL_WINDOW=egl or as one option.
Fixup of !1154 (merged)