Revert "Add compound-singleref (compound mode using single ref)"

This reverts commit ed69b09f598cb373db9d3bf86eae5ffae5e7b04b.

Change-Id: I3f9453596e42c2a2a78f8de7326cb267d32e17a5
diff --git a/configure b/configure
index 0a66c9f..dd4aef7 100755
--- a/configure
+++ b/configure
@@ -263,7 +263,6 @@
     interintra
     wedge
     compound_segment
-    compound_singleref
     ext_refs
     global_motion
     new_quant