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 https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1154

Part-of: <https://gitlab.freedesktop.org/gstreamer/gst-plugins-base/-/merge_requests/1169>

gst-libs/gst/gl/gstgldisplay.c

@@ -316,14 +316,12 @@ gst_gl_display_type_from_environment (void)
       return GST_GL_DISPLAY_TYPE_EGL;
     } else if (g_strstr_len (env, 4, "winrt")) {
       return GST_GL_DISPLAY_TYPE_EGL;
+    } else {
+      return GST_GL_DISPLAY_TYPE_NONE;
     }
   }
 
-  if (platform && g_strstr_len (platform, 3, "egl")) {
-    return GST_GL_DISPLAY_TYPE_EGL;
-  }
-
-  return GST_GL_DISPLAY_TYPE_NONE;
+  return GST_GL_DISPLAY_TYPE_ANY;
 }
 
 static GstGLDisplay *