Remove sub8x8_mc experimental flag

The re-designed sub8x8 coding block system settles. As part of the
research attempt, the sub8x8-mc experiment can now be removed.

BUG=aomedia:477

Change-Id: Icb1bf52471c560c1357e9121fbc5aa78ed854421

configure

diff --git a/configure b/configure
index 459b7ba..f3ad5ab 100755
--- a/configure
+++ b/configure
@@ -251,7 +251,6 @@
     compound_round
     ext_tx
     tx64x64
-    sub8x8_mc
     ext_intra
     intra_interp
     filter_intra